Universally Unique IDentifiers (UUID)

Universally Unique IDentifiers (UUID) Cisco Systems

kydavis@cisco.com

Uncloud

brad@peabody.io

University of Washington

pjl7@uw.edu

ART uuidrev uuid This specification defines the UUIDs (Universally Unique IDentifiers) and the UUID Uniform Resource Name (URN) namespace. UUIDs are also known as GUIDs (Globally Unique IDentifiers). A UUID is 128 bits long and is intended to guarantee uniqueness across space and time. UUIDs were originally used in the Apollo Network Computing System and later in the Open Software Foundation's (OSF) Distributed Computing Environment (DCE), and then in Microsoft Windows platforms. This specification is derived from the DCE specification with the kind permission of the OSF (now known as The Open Group). Information from earlier versions of the DCE specification have been incorporated into this document. This document obsoletes RFC4122.

Introduction This specification defines the UUIDs (Universally Unique IDentifiers) and the UUID Uniform Resource Name (URN) namespace. UUIDs are also known as GUIDs (Globally Unique IDentifiers). A UUID is 128 bits long and requires no central registration process. The use of UUIDs is extremely pervasive in computing. They comprise the core identifier infrastructure for many operating systems such as Microsoft Windows and applications such as the Mozilla Web browser and in many cases, become exposed in many non-standard ways. This specification attempts to standardize that practice as openly as possible and in a way that attempts to benefit the entire Internet. The information here is meant to be a concise guide for those wishing to implement services using UUIDs either in combination with URNs or otherwise. There is an ITU-T Recommendation and an ISO/IEC Standard that are derived from . Both sets of specifications have been aligned and are fully technically compatible. Nothing in this document should be construed to override the DCE standards that defined UUIDs.

Motivation One of the main reasons for using UUIDs is that no centralized authority is required to administer them (although two formats may leverage optional IEEE 802 node identifiers, others do not). As a result, generation on demand can be completely automated and used for a variety of purposes. The UUID generation algorithm described here supports very high allocation rates of 10 million per second per machine or more if necessary, so that they could even be used as transaction IDs. UUIDs are of a fixed size (128 bits), which is reasonably small compared to other alternatives. This lends itself well to sorting, ordering, and hashing of all sorts, storing in databases, simple allocation, and ease of programming in general. Since UUIDs are unique and persistent, they make excellent Uniform Resource Names. The unique ability to generate a new UUID without a registration process allows for UUIDs to be one of the URNs with the lowest minting cost.

Update Motivation Many things have changed in the time since UUIDs were originally created. Modern applications have a need to create and utilize UUIDs as the primary identifier for a variety of different items in complex computational systems, including but not limited to database keys, file names, machine or system names, and identifiers for event-driven transactions. One area in which UUIDs have gained popularity is database keys. This stems from the increasingly distributed nature of modern applications. In such cases, "auto increment" schemes often used by databases do not work well, as the effort required to coordinate sequential numeric identifiers across a network can easily become a burden. The fact that UUIDs can be used to create unique, reasonably short values in distributed systems without requiring coordination makes them a good alternative, but UUID versions 1-5, which were originally defined by , lack certain other desirable characteristics:

Non-time-ordered UUID versions such as UUIDv4 (described in ) have poor database index locality. This means that new values created in succession are not close to each other in the index and thus require inserts to be performed at random locations. The resulting negative performance effects on common structures used for this (B-tree and its variants) can be dramatic.
The 100-nanosecond Gregorian epoch used in UUIDv1 (described in ) timestamps is uncommon and difficult to represent accurately using a standard number format such as .
Introspection/parsing is required to order by time sequence, as opposed to being able to perform a simple byte-by-byte comparison.
Privacy and network security issues arise from using a MAC address in the node field of UUID version 1. Exposed MAC addresses can be used as an attack surface to locate network interfaces and reveal various other information about such machines (minimally manufacturer, potentially other details). Additionally, with the advent of virtual machines and containers, MAC address uniqueness is no longer guaranteed.
Many of the implementation details specified in involved trade offs that are neither possible to specify for all applications nor necessary to produce interoperable implementations.
did not distinguish between the requirements for generating a UUID and those for simply storing one, although they are often different.

Due to the aforementioned issues, many widely distributed database applications and large application vendors have sought to solve the problem of creating a better time-based, sortable unique identifier for use as a database key. This has led to numerous implementations over the past 10+ years solving the same problem in slightly different ways. While preparing this specification, the following 16 different implementations were analyzed for trends in total ID length, bit layout, lexical formatting/encoding, timestamp type, timestamp format, timestamp accuracy, node format/components, collision handling, and multi-timestamp tick generation sequencing:

by A. Feerasta
by Twitter
by Twitter
by Boundary
by Instagram
by Segment
by P. Pearcy
by T. Pawlak
by Sony
by IT. Cabrera
by R. Tallent
by A. Chilton
by Google
by O. Poitrey
by MongoDB
by E. Elliott

An inspection of these implementations and the issues described above has led to this document which intends to adapt new UUIDs to address these issues. Further, itself was in need an overhaul to address a number of topics such as but not limited to the following:

Miscellaneous erratas. Mostly around bit layout clarifications which lead to inconsistent implementations.
Decouple other UUID versions from UUIDv1 bit layout so that fields like "time_hi_and_version" do not need to be referenced within a non-time-based UUID while also providing "UUIDv1 like" definition sections for UUIDv3, UUIDv4, and UUIDv5.
Provide implementation best practices around many real-world scenarios and corner cases observed by existing and prototype implementations.
Update the document to address security best practices and considerations for the modern age as it pertains MAC addresses, hashing algorithms, secure randomness, and other topics.
Provide implementations a standard-based option for implementation specific and/or experimental UUID designs.
Provide more test vectors that illustrate real UUIDs created as per the specification.

Terminology

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Abbreviations The following abbreviations are used in this document:

UUID: Universally Unique Identifier
UUIDv1: Universally Unique Identifier Version 1
UUIDv2: Universally Unique Identifier Version 2
UUIDv3: Universally Unique Identifier Version 3
UUIDv4: Universally Unique Identifier Version 4
UUIDv5: Universally Unique Identifier Version 5
UUIDv6: Universally Unique Identifier Version 6
UUIDv7: Universally Unique Identifier Version 7
UUIDv8: Universally Unique Identifier Version 8
URN: Uniform Resource Names
ABNF: Augmented Backus-Naur Form
CSPRNG: Cryptographically Secure Pseudo-Random Number Generator
MAC: Media Access Control
MSB: Most Significant Bit
DBMS: Database Management System
IEEE: Institute of Electrical and Electronics Engineers, Inc.
ITU: International Telecommunication Union
MD5: Message Digest 5
SHA: Secure Hash Algorithm
SHA-1: Secure Hash Algorithm 1 with message digest of 160 bits
SHA-224: Secure Hash Algorithm 2 with message digest size of 224 bits
SHA-256: Secure Hash Algorithm 2 with message digest size of 256 bits
SHA-512: Secure Hash Algorithm 2 with message digest size of 512 bits
SHA-3: Secure Hash Algorithm 3
SHAKE: Secure Hash Algorithm 3 based on KECCAK algorithm
UTC: Coordinated Universal Time
OID: Object Identifier

Changelog draft-14

AD Review #2: IANA Subtype Modifications #170
AD Review #2: Specify Values for Variant/Subtype Column #171
AD Review #2: Grammar change at the end of 5.1 #172
SHA and Integer Verbiage clarifications #174
Disclaimer in nil/max that these do not fall into IETF variants #175
Fix v1/v6 final UUID in test vector #176

draft-13

Request IANA Registry #144
Describe allocation logic of Namespace ID #161
Move citation of Namesapce ID up to first instance in v3/v5 #167
Further normalize Namespace verbiage #166
Fix Timestamp of Time-Based UUIDv8 Example #164
Change RFC8937 and RFC4086 to Informative References #163
Discuss why v3/v5 are bad for Database Usage #155

draft-12

Typos #148 #156
SECDIR Review #141
SECDIR Review 2 #142
OPSDIR Review #145
INDIR Review 2 #140
IESG Grammar #146
Revise 16-bit MAC Node Usage #149
Add MSB3 to Variant Table #153
Additional Update Motivations #157
Expand v8 Time-based Example to larger timestamp #159
Fix Randomized Node value's mcast bit in Appendix #151
Clarify "Name-Based" is the same as "Hash-Based" #154
Move UUIDv8 Examples out of Test Vectors #150
Simplify UUIDv8 Hash-based Example #147

draft-11

Normalize "name space" to "namespace" everywhere #137
IANA Review: Verbiage to update RFC4122 references #134
DNSDIR re-review: Better Define "a canonical sequence of octets" #136
Crosspost: Typo in Approximate UUID timestamp calculations #135
INTDIR Review #139

draft-10

ARTART Review and Feedback #130
Clarify Hash Space IDs listed are not the only options #132
Add example to timestamp fuzzing #133

draft-09

Late addition of IETF reference for CSPRNG guidance #123
DNSDIR Review: Typos! #122
DNSDIR Review: DNS Considerations Update #121
Error in UUIDv8 Name-based Test Vector #129
Improve consistency of layout field definitions #128

draft-08

Fix typos #113
Fix errata 6225 (again) #117 #118
AD Review: BCP 14 - SHOULD #114
AD Review: Add proper references to v1 and v6 #116
AD Review: Remove SHOULD in section 4 #120
Discuss "front-loaded rollover counter" for 32-bit epoch with Padding method #115

draft-07

Even more grammar tweaks! #109
Remove unnecessary "32 bit" in UUIDv7 example #108
Change "fixed millisecond" -> "millisecond by default" relating to v7 #110
Revert Max UUID Naming #107
Author Changes

draft-06

More Grammar edits! #102
Tweak v7 description to de-emphasize optional components #103
Better Clarify Case in ABNF #104
Verbiage change in 6.2 #105

draft-05

Changed Max UUID to Max UUID to better complement Latin Nil UUID verbiage. #95
Align Method 3 text with the 12 bits limitation #96
Make Version/version casing consistent across 5. UUID Layouts #97
Cite MS COM GUID as little-endian #95

draft-04

Remove extra words #82, #88, and #93
Punctuation and minor style fixes #84
Change rounding mode of Method 4 Section 6.2 #90 (from #86)
Add verbal description of v7 generation to 5.7. UUID Version 7 #91
Remove Re-randomize Until Monotonic (Method 3) from Monotonicity and Counters #92
Fix ambiguous text around UUIDv6 clock sequence #89
Move endianness statement from layout to format section #85
Further modified abstract to separate URN topic from UUID definition #83
Provided three more UUID format examples #83
Added text further clarifying version construct is for the variant in this doc #83
Provided further clarification for local/global bit vs multicast bit #83

draft-03

Revised IANA Considerations #71
Fix "integral numbers of octets" verbiage #67
Transpose UUID Namespaces to match UUID Hashspaces #70
Reference all Hash Algorithms. #69
Normalize SHA abbreviation formats #66
Add other Hash Abbreviations #65
Remove URN from title #73
Move Community Considerations to Introduction #68
Move some Normative Reference to Informative #74
Misc formatting changes to address IDNITS feedback
Downgrade MUST NOT to SHOULD NOT for guessability of UUIDs #75
Misc. text formatting, typo fixes #78
Misc. text clarifications #79
Misc. SHOULD/MUST adjustments #80
Method 3 and 4 added to monotonic section #81

draft-02

Change md5_high in SHA-1 section to sha1_mid #59
Describe Nil/Max UUID in variant table #16
Further Clarify that non-descript node IDs are the preferred method in distributed UUID Generation #49
Appendix B, consistent naming #55
Remove duplicate ABNF from IANA considerations #56
Monotonic Error Checking missing newline #57
More Security Considerations Randomness #26
SHA-256 UUID Generation #50
Expand multiplexed fields within v1 and v6 bit definitions #43
Clean up text in UUIDs that Do Not Identify the Host #61
Revise UUID Generator States section #47
Expand upon why unix epoch rollover is not a problem #44
Delete Sample Code Appendix #62

draft-01

Mixed Case Spelling error #18
Add "UUIDs that Do Not Identify the Host as well" reference to security considerations #19
Out of Place Distributed node text #20
v6 clock_seq and node usage ambiguity #21
Figure 2 and 3 Fix Title #22
Move Namespace Registration Template to IANA Considerations #23
Verify ABNF formatting against RFC5234 #24
Bump ABNF reference to RFC 5234 #25
Modify v8 SHOULD NOT to MUST NOT #27
Remove "time-based" constraint from version 8 UUID #29
Further clarify v7 field description #125 #30
Typo: Section 4.2, Version Field, "UUID from in this" #33
Create better ABNF to represent Hex Digit #39
Break Binary form of UUID into two lines. #40
Move octet text from section 4 to section 5 #41
Add forward reference to UUIDv1 and UUIDv4 in Section 2 #42
Erroneous reference to v1 in monotonicity #45
Add Label for "Monotonic Error Checking" paragraph to frame the topic #46
Remove IEEE paragraph from "uuids that do not identify the host" #48
Grammar Review #52

draft-00

Merge RFC4122 with draft-peabody-dispatch-new-uuid-format-04.md
Change: Reference RFC1321 to RFC6151
Change: Reference RFC2141 to RFC8141
Change: Reference RFC2234 to RFC5234
Change: Reference FIPS 180-1 to FIPS 180-4 for SHA-1
Change: Converted UUIDv1 to match UUIDv6 section from Draft 04
Change: Trimmed down the ABNF representation
Change: http websites to https equivalent
Errata: Bad Reference to RFC1750 | 3641 #4
Errata: Change MD5 website to example.com | 3476 #6 (Also Fixes Errata: Fix uuid_create_md5_from_name() | 1352 #2)
Errata: Typo in code comment | 6665 #11
Errata: Fix BAD OID acronym | 6225 #9
Errata: Incorrect Parenthesis usage Section 4.3 | 184 #5
Errata: Lexicographically Sorting Paragraph Fix | 1428 #3
Errata: Fix 4.1.3 reference to the correct bits | 1957 #13
Errata: Fix reference to variant in octet 8 | 4975 #7
Errata: Further clarify 3rd/last bit of Variant for spec | 5560 #8
Errata: Fix clock_seq_hi_and_reserved most-significant bit verbiage | 4976 #10
Errata: Better Clarify network byte order when referencing most significant bits | 3546 #12
Draft 05: B.2. Example of a UUIDv7 Value two "var" in table #120
Draft 05: MUST verbiage in Reliability of 6.1 #121
Draft 05: Further discourage centralized registry for distributed UUID Generation.
New: Further Clarity of exact octet and bit of var/ver in this spec
New: Block diagram, bit layout, test vectors for UUIDv4
New: Block diagram, bit layout, test vectors for UUIDv3
New: Block diagram, bit layout, test vectors for UUIDv5
New: Add MD5 Security Considerations reference, RFC6151
New: Add SHA-1 Security Considerations reference, RFC6194

UUID Format The UUID format is 16 octets (128 bits) in size; the variant bits in conjunction with the version bits described in the next sections determine finer structure. While discussing UUID formats and layout, bit definitions start at 0 and end at 127 while octet definitions start at 0 and end at 15. In the absence of explicit application or presentation protocol specification to the contrary, each field is encoded with the Most Significant Byte first (known as network byte order). Saving UUIDs to binary format is done by sequencing all fields in big-endian format. However there is a known caveat that Microsoft's Component Object Model (COM) GUIDs leverage little-endian when saving GUIDs. The discussion of this is outside the scope of this specification. UUIDs MAY be represented as binary data or integers. When in use with URNs or as text in applications, any given UUID should be represented by the "hex-and-dash" string format consisting of multiple groups of upper or lowercase alphanumeric hexadecimal characters separated by single dashes/hyphens. When used with databases please refer to . The formal definition of the UUID string representation is provided by the following (ABNF) . Note that the alphabetic characters may be all uppercase, all lowercase, or mixed case, as per . An example UUID using this textual representation from the above ABNF is shown in .

Example String UUID format The same UUID from is represented in Binary (), Unsigned Integer () and as a URN () defined by .

Example Binary UUID

Example Unsigned Integer UUID (shown as a decimal number)

Example URN UUID There are many other ways to define a UUID format; some examples are detailed below. Please note that this is not an exhaustive list and is only provided for informational purposes.

Some UUID implementations, such as those found in and , will output UUID with the string format, including dashes, enclosed in curly braces.
provides UUID format definitions for use of UUID with an OID.
The legacy implementation produces a unique UUID format compatible with Variant 0xx of .

Variant Field The variant field determines the layout of the UUID. That is, the interpretation of all other bits in the UUID depends on the setting of the bits in the variant field. As such, it could more accurately be called a type field; we retain the original term for compatibility. The variant field consists of a variable number of the most significant bits of octet 8 of the UUID. lists the contents of the variant field, where the letter "x" indicates a "don't-care" value. UUID Variants

Msb0	Msb1	Msb2	Msb3	Variant	Description
0	x	x	x	1-7	Reserved, NCS backward compatibility and includes Nil UUID as per .
1	0	x	x	8-9,A-B	The variant specified in this document.
1	1	0	x	C-D	Reserved, Microsoft Corporation backward compatibility.
1	1	1	x	E-F	Reserved for future definition and includes Max UUID as per .

Interoperability, in any form, with variants other than the one defined here is not guaranteed but is not likely to be an issue in practice. Specifically for UUIDs in this document, bits 64 and 65 of the UUID (bits 0 and 1 of octet 8) MUST be set to 1 and 0 as specified in row 2 of . Accordingly, all bit and field layouts avoid the use of these bits.

Version Field The version number is in the most significant 4 bits of octet 6 (bits 48 through 51 of the UUID). lists all of the versions for this UUID variant 10xx specified in this document. UUID variant 10xx versions defined by this specification

Msb0	Msb1	Msb2	Msb3	Version	Description
0	0	0	0	0	Unused
0	0	0	1	1	The Gregorian time-based UUID specified in this document.
0	0	1	0	2	Reserved for DCE Security version, with embedded POSIX UUIDs.
0	0	1	1	3	The name-based version specified in this document that uses MD5 hashing.
0	1	0	0	4	The randomly or pseudo-randomly generated version specified in this document.
0	1	0	1	5	The name-based version specified in this document that uses SHA-1 hashing.
0	1	1	0	6	Reordered Gregorian time-based UUID specified in this document.
0	1	1	1	7	Unix Epoch time-based UUID specified in this document.
1	0	0	0	8	Reserved for custom UUID formats specified in this document.
1	0	0	1	9	Reserved for future definition.
1	0	1	0	10	Reserved for future definition.
1	0	1	1	11	Reserved for future definition.
1	1	0	0	12	Reserved for future definition.
1	1	0	1	13	Reserved for future definition.
1	1	1	0	14	Reserved for future definition.
1	1	1	1	15	Reserved for future definition.

An example version/variant layout for UUIDv4 follows the table where M represents the version placement for the hexadecimal representation of 0x4 (0b0100) and the N represents the variant placement for one of the four possible hexadecimal representation of variant 10xx: 0x8 (0b1000), 0x9 (0b1001), 0xA (0b1010), 0xB (0b1011)

UUIDv4 Variant Examples It should be noted that the other remaining UUID variants found in leverage different sub-typing/versioning mechanisms. The recording and definition of the remaining UUID variant and sub-typing combinations are outside of the scope of this document.

UUID Layouts To minimize confusion about bit assignments within octets and among differing versions, the UUID record definition is provided as a grouping of fields within a bit layout consisting of four octets per row. The fields are presented with the most significant one first.

UUID Version 1 UUID version 1 is a time-based UUID featuring a 60 bit timestamp represented by Coordinated Universal Time (UTC) as a count of 100- nanosecond intervals since 00:00:00.00, 15 October 1582 (the date of Gregorian reform to the Christian calendar). UUIDv1 also features a clock sequence field which is used to help avoid duplicates that could arise when the clock is set backwards in time or if the node ID changes. The node field consists of an IEEE 802 MAC address, usually the host address or a randomly derived value per and .

UUIDv1 Field and Bit Layout

time_low:: The least significant 32 bits of the 60 bit starting timestamp. Occupies bits 0 through 31 (octets 0-3).
time_mid:: The middle 16 bits of the 60 bit starting timestamp. Occupies bits 32 through 47 (octets 4-5).
ver:: The 4 bit version field as defined by , set to 0b0001 (1). Occupies bits 48 through 51 of octet 6.
time_high:: 12 bits that will contain the most significant 12 bits from the 60 bit starting timestamp. Occupies bits 52 through 63 (octets 6-7).
var:: The 2 bit variant field as defined by , set to 0b10. Occupies bits 64 and 65 of octet 8.
clock_seq:: The 14 bits containing the clock sequence. Occupies bits 66 through 79 (octets 8-9).
node:: 48 bit spatially unique identifier. Occupies bits 80 through 127 (octets 10-15).

For systems that do not have UTC available, but do have the local time, they may use that instead of UTC, as long as they do so consistently throughout the system. However, this is not recommended since generating the UTC from local time only needs a time zone offset. If the clock is set backwards, or might have been set backwards (e.g., while the system was powered off), and the UUID generator can not be sure that no UUIDs were generated with timestamps larger than the value to which the clock was set, then the clock sequence MUST be changed. If the previous value of the clock sequence is known, it MAY be incremented; otherwise it SHOULD be set to a random or high-quality pseudo-random value. Similarly, if the node ID changes (e.g., because a network card has been moved between machines), setting the clock sequence to a random number minimizes the probability of a duplicate due to slight differences in the clock settings of the machines. If the value of the clock sequence associated with the changed node ID were known, then the clock sequence MAY be incremented, but that is unlikely. The clock sequence MUST be originally (i.e., once in the lifetime of a system) initialized to a random number to minimize the correlation across systems. This provides maximum protection against node identifiers that may move or switch from system to system rapidly. The initial value MUST NOT be correlated to the node identifier. Notes about IEEE 802 derived nodes:

On systems with multiple IEEE 802 addresses, any available one MAY be used.
On systems with no IEEE address, a randomly or pseudo-randomly generated value MUST be used; see and .
On systems utilizing a 64 bit MAC address the least significant, right-most 48 bits MAY be used.
On systems utilizing an IEEE 802.15.4 16 bit address SHOULD instead utilize their 64 bit MAC address where least significant, right-most 48 bits MAY be used. An alternative is to generate 32 bits of random data and postfix at the end of the 16 bit MAC address to create a 48 bit value.

UUID Version 2 UUID version 2 is known as DCE Security UUIDs and . As such, the definition of these UUIDs is outside the scope of this specification.

UUID Version 3 UUID version 3 is meant for generating UUIDs from "names" that are drawn from, and unique within, some "namespace" as per . UUIDv3 values are created by computing an MD5 hash over a given namespace ID value () concatenated with the desired name value after both have been converted to a canonical sequence of octets, as defined by the standards or conventions of its namespace, in network byte order. This MD5 value is then used to populate all 128 bits of the UUID layout. The UUID version and variant then replace the respective bits as defined by and . An example of this bit substitution can be found in . Information around selecting a desired name's canonical format within a given namespace can be found in , "A note on names". Where possible UUIDv5 SHOULD be used in lieu of UUIDv3. For more information on MD5 security considerations see .

UUIDv3 Field and Bit Layout

md5_high:: The first 48 bits of the layout are filled with the most significant, left-most 48 bits from the computed MD5 value. Occupies bits 0 through 47 (octets 0-5).
ver:: The 4 bit version field as defined by , set to 0b0011 (3). Occupies bits 48 through 51 of octet 6.
md5_mid:: 12 more bits of the layout consisting of the least significant, right-most 12 bits of 16 bits immediately following md5_high from the computed MD5 value. Occupies bits 52 through 63 (octets 6-7).
var:: The 2 bit variant field as defined by , set to 0b10. Occupies bits 64 and 65 of octet 8.
md5_low:: The final 62 bits of the layout immediately following the var field to be filled with the least-significant, right-most bits of the final 64 bits from the computed MD5 value. Occupies bits 66 through 127 (octets 8-15)

UUID Version 4 UUID version 4 is meant for generating UUIDs from truly-random or pseudo-random numbers. An implementation may generate 128 bits of random data which is used to fill out the UUID fields in . The UUID version and variant then replace the respective bits as defined by and . Alternatively, an implementation MAY choose to randomly generate the exact required number of bits for random_a, random_b, and random_c (122 bits total), and then concatenate the version and variant in the required position. For guidelines on random data generation see .

UUIDv4 Field and Bit Layout

random_a:: The first 48 bits of the layout that can be filled with random data as specified in . Occupies bits 0 through 47 (octets 0-5).
ver:: The 4 bit version field as defined by , set to 0b0100 (4). Occupies bits 48 through 51 of octet 6.
random_b:: 12 more bits of the layout that can be filled random data as per . Occupies bits 52 through 63 (octets 6-7).
var:: The 2 bit variant field as defined by , set to 0b10. Occupies bits 64 and 65 of octet 8.
random_c:: The final 62 bits of the layout immediately following the var field to be filled with random data as per . Occupies bits 66 through 127 (octets 8-15).

UUID Version 5 UUID version 5 is meant for generating UUIDs from "names" that are drawn from, and unique within, some "namespace" as per . UUIDv5 values are created by computing an SHA-1 hash over a given namespace ID value () concatenated with the desired name value after both have been converted to a canonical sequence of octets, as defined by the standards or conventions of its namespace, in network byte order. The most significant, left-most 128 bits of the SHA-1 value is then used to populate all 128 bits of the UUID layout and the remaining 32 least significant, right-most bits of SHA-1 output are discarded. The UUID version and variant then replace the respective bits as defined by and . An example of this bit substitution and discarding excess bits can be found in . Information around selecting a desired name's canonical format within a given namespace can be found in , "A note on names". There may be scenarios, usually depending on organizational security policies, where SHA-1 libraries may not be available or deemed unsafe for use. As such, it may be desirable to generate name-based UUIDs derived from SHA-256 or newer SHA methods. These name-based UUIDs MUST NOT utilize UUIDv5 and MUST be within the UUIDv8 space defined by . An illustrative example of UUIDv8 for SHA-256 name-based UUIDs is provided in the appendix . For more information on SHA-1 security considerations see .

UUIDv5 Field and Bit Layout

sha1_high:: The first 48 bits of the layout are filled with the most significant, left-most 48 bits from the computed SHA-1 value. Occupies bits 0 through 47 (octets 0-5).
ver:: The 4 bit version field as defined by , set to 0b0101 (5). Occupies bits 48 through 51 of octet 6.
sha1_mid:: 12 more bits of the layout consisting of the least significant, right-most 12 bits of 16 bits immediately following sha1_high from the computed SHA-1 value. Occupies bits 52 through 63 (octets 6-7).
var:: The 2 bit variant field as defined by , set to 0b10. Occupies bits 64 and 65 of octet 8.
sha1_low:: The final 62 bits of the layout immediately following the var field to be filled by skipping the 2 most significant, left-most bits of the remaining SHA-1 hash and then using the next 62 most significant, left-most bits. Any leftover SHA-1 bits are discarded and unused. Occupies bits 66 through 127 (octets 8-15).

UUID Version 6 UUID version 6 is a field-compatible version of UUIDv1 , reordered for improved DB locality. It is expected that UUIDv6 will primarily be used in contexts where UUIDv1 is used. Systems that do not involve legacy UUIDv1 SHOULD use UUIDv7 instead. Instead of splitting the timestamp into the low, mid, and high sections from UUIDv1, UUIDv6 changes this sequence so timestamp bytes are stored from most to least significant. That is, given a 60 bit timestamp value as specified for UUIDv1 in , for UUIDv6, the first 48 most significant bits are stored first, followed by the 4 bit version (same position), followed by the remaining 12 bits of the original 60 bit timestamp. The clock sequence and node bits remain unchanged from their position in . The clock sequence and node bits SHOULD be reset to a pseudo-random value for each new UUIDv6 generated; however, implementations MAY choose to retain the old clock sequence and MAC address behavior from . For more information on MAC address usage within UUIDs see the . The format for the 16-byte, 128 bit UUIDv6 is shown in .

UUIDv6 Field and Bit Layout

time_high:: The most significant 32 bits of the 60 bit starting timestamp. Occupies bits 0 through 31 (octets 0-3).
time_mid:: The middle 16 bits of the 60 bit starting timestamp. Occupies bits 32 through 47 (octets 4-5).
ver:: The 4 bit version field as defined by , set to 0b0110 (6). Occupies bits 48 through 51 of octet 6.
time_low:: 12 bits that will contain the least significant 12 bits from the 60 bit starting timestamp. Occupies bits 52 through 63 (octets 6-7).
var:: The 2 bit variant field as defined by , set to 0b10. Occupies bits 64 and 65 of octet 8.
clock_seq:: The 14 bits containing the clock sequence. Occupies bits 66 through 79 (octets 8-9).
node:: 48 bit spatially unique identifier. Occupies bits 80 through 127 (octets 10-15).

With UUIDv6, the steps for splitting the timestamp into time_high and time_mid are OPTIONAL since the 48 bits of time_high and time_mid will remain in the same order. An extra step of splitting the first 48 bits of the timestamp into the most significant 32 bits and least significant 16 bits proves useful when reusing an existing UUIDv1 implementation.

UUID Version 7 UUID version 7 features a time-ordered value field derived from the widely implemented and well known Unix Epoch timestamp source, the number of milliseconds since midnight 1 Jan 1970 UTC, leap seconds excluded. UUIDv7 generally has improved entropy characteristics over UUIDv1 or UUIDv6 . UUIDv7 values are created by allocating a Unix timestamp in milliseconds in the most significant 48 bits and filling the remaining 74 bits, excluding the required version and variant bits, with random bits for each new UUIDv7 generated to provide uniqueness as per . Alternatively, implementations MAY fill the 74 bits, jointly, with a combination of the following subfields, in this order from the most significant bits to the least, to guarantee additional monotonicity within a millisecond:

An OPTIONAL sub-millisecond timestamp fraction (12 bits at maximum) as per (Method 3).
An OPTIONAL carefully seeded counter as per (Method 1 or 2).
Random data for each new UUIDv7 generated for any remaining space.

Implementations SHOULD utilize UUIDv7 instead of UUIDv1 and UUIDv6 if possible.

UUIDv7 Field and Bit Layout

unix_ts_ms:: 48 bit big-endian unsigned number of Unix epoch timestamp in milliseconds as per . Occupies bits 0 through 47 (octets 0-5).
ver:: The 4 bit version field as defined by , set to 0b0111 (7). Occupies bits 48 through 51 of octet 6.
rand_a:: 12 bits pseudo-random data to provide uniqueness as per and/or optional constructs to guarantee additional monotonicity as per . Occupies bits 52 through 63 (octets 6-7).
var:: The 2 bit variant field as defined by , set to 0b10. Occupies bits 64 and 65 of octet 8.
rand_b:: The final 62 bits of pseudo-random data to provide uniqueness as per and/or an optional counter to guarantee additional monotonicity as per . Occupies bits 66 through 127 (octets 8-15).

UUID Version 8 UUID version 8 provides an RFC-compatible format for experimental or vendor-specific use cases. The only requirement is that the variant and version bits MUST be set as defined in and . UUIDv8's uniqueness will be implementation-specific and MUST NOT be assumed. The only explicitly defined bits are those of the version and variant fields, leaving 122 bits for implementation specific UUIDs. To be clear: UUIDv8 is not a replacement for UUIDv4 where all 122 extra bits are filled with random data. Some example situations in which UUIDv8 usage could occur:

An implementation would like to embed extra information within the UUID other than what is defined in this document.
An implementation has other application/language restrictions which inhibit the use of one of the current UUIDs.

The appendix, , provides two illustrative examples of custom UUIDv8 algorithms to address two example scenarios.

UUIDv8 Field and Bit Layout

custom_a:: The first 48 bits of the layout that can be filled as an implementation sees fit. Occupies bits 0 through 47 (octets 0-5).
ver:: The 4 bit version field as defined by , set to 0b1000 (8). Occupies bits 48 through 51 of octet 6.
custom_b:: 12 more bits of the layout that can be filled as an implementation sees fit. Occupies bits 52 through 63 (octets 6-7).
var:: The 2 bit variant field as defined by , set to 0b10. Occupies bits 64 and 65 of octet 8.
custom_c:: The final 62 bits of the layout immediately following the var field to be filled as an implementation sees fit. Occupies bits 66 through 127 (octets 8-15).

Nil UUID The nil UUID is special form of UUID that is specified to have all 128 bits set to zero.

Nil UUID Format A Nil UUID value can be useful to communicate the absence of any other UUID value in situations that otherwise require or use a 128 bit UUID. A Nil UUID can express the concept "no such value here". Thus it is reserved for such use as needed for implementation-specific situations. Note that the Nil UUID value falls within the range of the Apollo NCS variant as per the first row of rather than the variant defined by this document.

Max UUID The Max UUID is a special form of UUID that is specified to have all 128 bits set to 1. This UUID can be thought of as the inverse of Nil UUID defined in .

Max UUID Format A Max UUID value can be used as a sentinel value in situations where a 128 bit UUID is required but a concept such as "end of UUID list" needs to be expressed, and is reserved for such use as needed for implementation-specific situations. Note that the Max UUID value falls within the range of the "yet to be defined" future UUID variant as per the last row of rather than the variant defined by this document.

UUID Best Practices The minimum requirements for generating UUIDs are described in this document for each version. Everything else is an implementation detail and it is up to the implementer to decide what is appropriate for a given implementation. Various relevant factors are covered below to help guide an implementer through the different trade-offs among differing UUID implementations.

Timestamp Considerations UUID timestamp source, precision, and length was the topic of great debate while creating UUIDv7 for this specification. Choosing the right timestamp for your application is a very important topic. This section will detail some of the most common points on this topic.

Reliability:: Implementations acquire the current timestamp from a reliable source to provide values that are time-ordered and continually increasing. Care must be taken to ensure that timestamp changes from the environment or operating system are handled in a way that is consistent with implementation requirements. For example, if it is possible for the system clock to move backward due to either manual adjustment or corrections from a time synchronization protocol, implementations need to determine how to handle such cases. (See Altering, Fuzzing, or Smearing below.)
Source:: UUID version 1 and 6 both utilize a Gregorian epoch timestamp while UUIDv7 utilizes a Unix Epoch timestamp. If other timestamp sources or a custom timestamp epoch are required, UUIDv8 MUST be used.
Sub-second Precision and Accuracy:: Many levels of precision exist for timestamps: milliseconds, microseconds, nanoseconds, and beyond. Additionally fractional representations of sub-second precision may be desired to mix various levels of precision in a time-ordered manner. Furthermore, system clocks themselves have an underlying granularity and it is frequently less than the precision offered by the operating system. With UUID version 1 and 6, 100-nanoseconds of precision are present while UUIDv7 features millisecond level of precision by default within the Unix epoch that does not exceed the granularity capable in most modern systems. For other levels of precision UUIDv8 is available. Similar to , with UUIDv1 or UUIDv6, a high resolution timestamp can be simulated by keeping a count of the number of UUIDs that have been generated with the same value of the system time, and using it to construct the low order bits of the timestamp. The count of the high resolution timestamp will range between zero and the number of 100-nanosecond intervals per system time interval.
Length:: The length of a given timestamp directly impacts how many timestamp ticks can be contained in a UUID before the maximum value for the timestamp field is reached. Take care to ensure that the proper length is selected for a given timestamp. UUID version 1 and 6 utilize a 60 bit timestamp valid until 5623 AD and UUIDv7 features a 48 bit timestamp valid until the year 10889 AD.
Altering, Fuzzing, or Smearing:: Implementations MAY alter the actual timestamp. Some examples include security considerations around providing a real clock value within a UUID, to correct inaccurate clocks, to handle leap seconds, or instead of dividing a number of microseconds by 1000 to obtain a millisecond value; dividing by 1024 (or some other value) for performance reasons. This specification makes no requirement or guarantee about how close the clock value needs to be to the actual time. If UUIDs do not need to be frequently generated, the UUIDv1 or UUIDv6 timestamp can simply be the system time multiplied by the number of 100-nanosecond intervals per system time interval.
Padding:: When timestamp padding is required, implementations MUST pad the most significant bits (left-most) bits with data. An example for this padding data is to fill the most significant, left-most bits of a Unix timestamp with zeroes to complete the 48 bit timestamp in UUIDv7. An alternative approach for padding data is to fill the most significant, left-most bits with the number of 32 bit Unix timestamp roll-overs after 2038-01-19.
Truncating:: When timestamps need to be truncated, the lower, least significant bits MUST be used. An example would be truncating a 64 bit Unix timestamp to the least significant, right-most 48 bits for UUIDv7.
Error Handling:: If a system overruns the generator by requesting too many UUIDs within a single system time interval, the UUID service can return an error, or stall the UUID generator until the system clock catches up, and MUST NOT return knowingly duplicate values due to a counter rollover. Note that if the processors overrun the UUID generation frequently, additional node identifiers can be allocated to the system, which will permit higher speed allocation by making multiple UUIDs potentially available for each time stamp value. Similar techniques are discussed in .

Monotonicity and Counters Monotonicity (each subsequent value being greater than the last) is the backbone of time-based sortable UUIDs. Normally, time-based UUIDs from this document will be monotonic due to an embedded timestamp; however, implementations can guarantee additional monotonicity via the concepts covered in this section. Take care to ensure UUIDs generated in batches are also monotonic. That is, if one thousand UUIDs are generated for the same timestamp, there should be sufficient logic for organizing the creation order of those one thousand UUIDs. Batch UUID creation implementations MAY utilize a monotonic counter that increments for each UUID created during a given timestamp. For single-node UUID implementations that do not need to create batches of UUIDs, the embedded timestamp within UUID version 6 and 7 can provide sufficient monotonicity guarantees by simply ensuring that timestamp increments before creating a new UUID. Distributed nodes are discussed in . Implementations SHOULD employ the following methods for single-node UUID implementations that require batch UUID creation, or are otherwise concerned about monotonicity with high frequency UUID generation.

Fixed-Length Dedicated Counter Bits (Method 1):: Some implementations allocate a specific number of bits in the UUID layout to the sole purpose of tallying the total number of UUIDs created during a given UUID timestamp tick. A fixed bit-length counter, if present, MUST be positioned immediately after the embedded timestamp. This promotes sortability and allows random data generation for each counter increment. With this method, the rand_a section (or a subset of its left-most bits) of UUIDv7 is used as fixed-length dedicated counter bits that are incremented for every UUID generation. The trailing random bits generated for each new UUID in rand_b can help produce unguessable UUIDs. In the event more counter bits are required, the most significant (left-most) bits of rand_b MAY be used as additional counter bits.
Monotonic Random (Method 2):: With this method, the random data is extended to also function as a counter. This monotonic value can be thought of as a "randomly seeded counter" which MUST be incremented in the least significant position for each UUID created on a given timestamp tick. UUIDv7's rand_b section SHOULD be utilized with this method to handle batch UUID generation during a single timestamp tick. The increment value for every UUID generation is a random integer of any desired length larger than zero. It ensures the UUIDs retain the required level of unguessability provided by the underlying entropy. The increment value MAY be 1 when the number of UUIDs generated in a particular period of time is important and guessability is not an issue. However, incrementing the counter by 1 SHOULD NOT be used by implementations that favor unguessability, as the resulting values are easily guessable.
Replace Left-Most Random Bits with Increased Clock Precision (Method 3):: For UUIDv7, which has millisecond timestamp precision, it is possible to use additional clock precision available on the system to substitute for up to 12 random bits immediately following the timestamp. This can provide values that are time-ordered with sub-millisecond precision, using however many bits are appropriate in the implementation environment. With this method, the additional time precision bits MUST follow the timestamp as the next available bit, in the rand_a field for UUIDv7. To calculate this value, start with the portion of the timestamp expressed as a fraction of clock's tick value (fraction of a millisecond for UUIDv7). Compute the count of possible values that can be represented in the available bit space, 4096 for the UUIDv7 rand_a field. Using floating point or scaled integer arithmetic, multiply this fraction of a millisecond value by 4096 and round down (toward zero) to an integer result to arrive at a number between 0 and the maximum allowed for the indicated bits which sorts monotonically based on time. Each increasing fractional value will result in an increasing bit field value, to the precision available with these bits. For example, let's assume a system timestamp of 1 Jan 2023 12:34:56.1234567. Taking the precision greater than 1ms gives us a value of 0.4567, as a fraction of a millisecond. If we wish to encode this as 12 bits, we can take the count of possible values that fit in those bits (4096, or 2 to the 12th power) and multiply it by our millisecond fraction value of 0.4567 and truncate the result to an integer, which gives an integer value of 1870. Expressed as hexadecimal it is 0x74E, or the binary bits 0b011101001110. One can then use those 12 bits as the most significant (left-most) portion of the random section of the UUID (e.g., the rand_a field in UUIDv7). This works for any desired bit length that fits into a UUID, and applications can decide the appropriate length based on available clock precision, but for UUIDv7, it is limited to 12 bits at maximum to reserve sufficient space for random bits. The main benefit to encoding additional timestamp precision is that it utilizes additional time precision already available in the system clock to provide values that are more likely to be unique, and thus may simplify certain implementations. This technique can also be used in conjunction with one of the other methods, where this additional time precision would immediately follow the timestamp, and then if any bits are to be used as clock sequence they would follow next.

The following sub-topics cover topics related solely with creating reliable fixed-length dedicated counters:

Fixed-Length Dedicated Counter Seeding:: Implementations utilizing the fixed-length counter method randomly initialize the counter with each new timestamp tick. However, when the timestamp has not increased, the counter is instead incremented by the desired increment logic. When utilizing a randomly seeded counter alongside Method 1, the random value MAY be regenerated with each counter increment without impacting sortability. The downside is that Method 1 is prone to overflows if a counter of adequate length is not selected or the random data generated leaves little room for the required number of increments. Implementations utilizing fixed-length counter method MAY also choose to randomly initialize a portion of the counter rather than the entire counter. For example, a 24 bit counter could have the 23 bits in least-significant, right-most, position randomly initialized. The remaining most significant, left-most counter bit is initialized as zero for the sole purpose of guarding against counter rollovers.
Fixed-Length Dedicated Counter Length:: Select a counter bit-length that can properly handle the level of timestamp precision in use. For example, millisecond precision generally requires a larger counter than a timestamp with nanosecond precision. General guidance is that the counter SHOULD be at least 12 bits but no longer than 42 bits. Care must be taken to ensure that the counter length selected leaves room for sufficient entropy in the random portion of the UUID after the counter. This entropy helps improve the unguessability characteristics of UUIDs created within the batch.

The following sub-topics cover rollover handling with either type of counter method:

Counter Rollover Guards:: The technique from Fixed-Length Dedicated Counter Seeding that describes allocating a segment of the fixed-length counter as a rollover guard is also helpful to mitigate counter rollover issues. This same technique can be used with monotonic random counter methods by ensuring that the total length of a possible increment in the least significant, right most position is less than the total length of the random value being incremented. As such, the most significant, left-most, bits can be incremented as rollover guarding.
Counter Rollover Handling:: Counter rollovers MUST be handled by the application to avoid sorting issues. The general guidance is that applications that care about absolute monotonicity and sortability should freeze the counter and wait for the timestamp to advance which ensures monotonicity is not broken. Alternatively, implementations MAY increment the timestamp ahead of the actual time and reinitialize the counter.

Implementations MAY use the following logic to ensure UUIDs featuring embedded counters are monotonic in nature:

Compare the current timestamp against the previously stored timestamp.
If the current timestamp is equal to the previous timestamp, increment the counter according to the desired method.
If the current timestamp is greater than the previous timestamp, re-initialize the desired counter method to the new timestamp and generate new random bytes (if the bytes were frozen or being used as the seed for a monotonic counter).

Monotonic Error Checking:: Implementations SHOULD check if the currently generated UUID is greater than the previously generated UUID. If this is not the case then any number of things could have occurred, such as clock rollbacks, leap second handling, and counter rollovers. Applications SHOULD embed sufficient logic to catch these scenarios and correct the problem to ensure that the next UUID generated is greater than the previous, or at least report an appropriate error. To handle this scenario, the general guidance is that application MAY reuse the previous timestamp and increment the previous counter method.

UUID Generator States The (optional) UUID generator state only needs to be read from stable storage once at boot time, if it is read into a system-wide shared volatile store (and updated whenever the stable store is updated). This stable storage MAY be used to record various portions of the UUID generation which prove useful for batch UUID generation purposes and monotonic error checking with UUIDv6 and UUIDv7. These stored values include but are not limited to last known timestamp, clock sequence, counters, and random data. If an implementation does not have any stable store available, then it MAY proceed with UUID generation as if this was the first UUID created within a batch. This is the least desirable implementation because it will increase the frequency of creation of values such as clock sequence, counters, or random data, which increases the probability of duplicates. Further, frequent generation of random numbers also puts more stress on any entropy source and or entropy pool being used as the basis for such random numbers. An implementation MAY also return an application error in the event that collision resistance is of the utmost concern. The semantics of this error are up to the application and implementation. See for more information on weighting collision tolerance in applications. For UUIDv1 and UUIDv6, if the node ID can never change (e.g., the network interface card from which the node ID is derived is inseparable from the system), or if any change also re-initializes the clock sequence to a random value, then instead of keeping it in stable store, the current node ID may be returned. For UUIDv1 and UUIDv6, the state does not always need to be written to stable store every time a UUID is generated. The timestamp in the stable store can be periodically set to a value larger than any yet used in a UUID. As long as the generated UUIDs have timestamps less than that value, and the clock sequence and node ID remain unchanged, only the shared volatile copy of the state needs to be updated. Furthermore, if the timestamp value in stable store is in the future by less than the typical time it takes the system to reboot, a crash will not cause a re-initialization of the clock sequence. If it is too expensive to access shared state each time a UUID is generated, then the system-wide generator can be implemented to allocate a block of time stamps each time it is called; a per- process generator can allocate from that block until it is exhausted.

Distributed UUID Generation Some implementations MAY desire to utilize multi-node, clustered, applications which involve two or more nodes independently generating UUIDs that will be stored in a common location. While UUIDs already feature sufficient entropy to ensure that the chances of collision are low, as the total number of UUID generating nodes increase; so does the likelihood of a collision. This section will detail the two additional collision resistance approaches that have been observed by multi-node UUID implementations in distributed environments. It should be noted that, although this section details two methods for the sake of completeness, implementations should utilize the pseudo-random Node ID option if additional collision resistance for distributed UUID generation is a requirement. Likewise, utilization of either method is not required for implementing UUID generation in distributed environments.

Node IDs:: With this method, a pseudo-random Node ID value is placed within the UUID layout. This identifier helps ensure the bit-space for a given node is unique, resulting in UUIDs that do not conflict with any other UUID created by another node with a different node id. Implementations that choose to leverage an embedded node id SHOULD utilize UUIDv8. The node id SHOULD NOT be an IEEE 802 MAC address as per . The location and bit length are left to implementations and are outside the scope of this specification. Furthermore, the creation and negotiation of unique node ids among nodes is also out of scope for this specification.
Centralized Registry:: With this method all nodes tasked with creating UUIDs consult a central registry and confirm the generated value is unique. As applications scale, the communication with the central registry could become a bottleneck and impact UUID generation in a negative way. Shared knowledge schemes with central/global registries are outside the scope of this specification and is NOT RECOMMENDED.

Distributed applications generating UUIDs at a variety of hosts MUST be willing to rely on the random number source at all hosts.

Name-Based UUID Generation Although some prefer to use the word "hash-based" to describe UUIDs featuring hashing algorithms (MD5 or SHA-1), this document retains the usage of the adjective "name-based" in order to maintain consistency with historical documents and existing implementations. The requirements for name-based UUIDs are as follows:

UUIDs generated at different times from the same name (using the same canonical format) in the same namespace MUST be equal.
UUIDs generated from two different names (same or differing canonical format) in the same namespace should be different (with very high probability).
UUIDs generated from the same name (same or differing canonical format) in two different namespaces should be different (with very high probability).
If two UUIDs that were generated from names (using the same canonical format) are equal, then they were generated from the same name in the same namespace (with very high probability).

A note on names:: The concept of name (and namespace) should be broadly construed and not limited to textual names. A canonical sequence of octets is one that conforms to the specification for that name form's canonical representation. A name can have many usual forms, only one of which can be canonical. An implementer of new namespaces for UUIDs needs to reference the specification for the canonical form of names in that space, or define such a canonical for the namespace if it does not exist. For example, at the time of this specification, domain name system (DNS) has three conveyance formats: common (www.example.com), presentation (www.example.com.) and wire format (3www7example3com0). Looking at distinguished names (DNs), the previous version of this specification allowed either text based or binary distinguished encoding rules (DER) based names as inputs. For uniform resource locators (URLs), one could provide a fully-qualified domain-name (FQDN) with or without the protocol identifier (www.example.com) or (https://www.example.com). When it comes to object identifiers (OIDs) one could choose dot-notation without the leading dot (2.999), choose to include the leading dot (.2.999) or select one of the many formats from such as OID Internationalized Resource Identifier (OID-IRI) (/Joint-ISO-ITU-T/Example). While most users may default to the common format for DNS, FQDN format for a URL, text format for X.500 and dot-notation without a leading dot for OID; name-based UUID implementations generally SHOULD allow arbitrary input which will compute name-based UUIDs for any of the aforementioned example names and others not defined here. Each name format within a namespace will output different UUIDs. As such, the mechanisms or conventions used for allocating names and ensuring their uniqueness within their namespaces are beyond the scope of this specification.

Namespace ID Usage and Allocation This section and table, , details the namespace IDs for some potentially interesting namespaces such those for domain name system (DNS), uniform resource locators (URLs), object identifiers (OIDs), and distinguished names (DNs). Further, this section also details allocation, IANA registration and other details pertinent to Namespace IDs. IANA may use the table as-is replacing "This Document" replaced as required. Namespace IDs

Namespace	Namespace ID Value	Namespace ID Reference
DNS	6ba7b810-9dad-11d1-80b4-00c04fd430c8	, This document
URL	6ba7b811-9dad-11d1-80b4-00c04fd430c8	, This document
OID	6ba7b812-9dad-11d1-80b4-00c04fd430c8	, This document
X500	6ba7b814-9dad-11d1-80b4-00c04fd430c8	, This document

Items may be added to this table using "Specification Required" policy as per . For designated experts, generally speaking, Namespace IDs are allocated as follows:

The first namespace ID value, for DNS, was calculated from a time-based UUIDv1 and "6ba7b810-9dad-11d1-80b4-00c04fd430c8" used as a starting point.
Subsequent namespace ID values increment the least-significant, right-most bit of time_low "6ba7b810" while freezing the rest of the UUID to "9dad-11d1-80b4-00c04fd430c8".
New namespace ID values MUST use this same logic and MUST NOT use a previously used Namespace ID value.
Thus, "6ba7b815" is the next available time_low for a new Namespace ID value with the full ID being "6ba7b815-9dad-11d1-80b4-00c04fd430c8".
The upper bound for time_low in this special use, namespace ID values, is "ffffffff" or "ffffffff-9dad-11d1-80b4-00c04fd430c8" which should be sufficient space for future namespace ID values.

Note that the namespace ID value "6ba7b813-9dad-11d1-80b4-00c04fd430c8" and its usage is not defined by this document or by , as such it SHOULD NOT be used as a Namespace ID value. New Namespace ID values MUST be documented as per if they are to be globally available and fully interoperable. Implementations MAY continue to use vendor-specific, application-specific, and deployment-specific Namespace ID values but know that interoperability is not guaranteed. These custom Namespace ID values MUST NOT use the logic above and instead are RECOMMENDED to generate a UUIDv4 or UUIDv7 Namespace ID value. If collision probability () and uniqueness () of the final name-based UUID are not a problem; an implementation MAY also leverage UUIDv8 instead to create a custom, application-specific Namespace ID value. Implementations SHOULD provide the ability to input a custom namespace to account for newly registered IANA Namespace ID values outside of those listed in this section or custom, application specific Namespace ID values.

Collision Resistance Implementations should weigh the consequences of UUID collisions within their application and when deciding between UUID versions that use entropy (randomness) versus the other components such as those in and . This is especially true for distributed node collision resistance as defined by . There are two example scenarios below which help illustrate the varying seriousness of a collision within an application.

Low Impact:: A UUID collision generated a duplicate log entry which results in incorrect statistics derived from the data. Implementations that are not negatively affected by collisions may continue with the entropy and uniqueness provided by the traditional UUID format.
High Impact:: A duplicate key causes an airplane to receive the wrong course which puts people's lives at risk. In this scenario there is no margin for error. Collisions must be avoided and failure is unacceptable. Applications dealing with this type of scenario must employ as much collision resistance as possible within the given application context.

Global and Local Uniqueness UUIDs created by this specification MAY be used to provide local uniqueness guarantees. For example, ensuring UUIDs created within a local application context are unique within a database MAY be sufficient for some implementations where global uniqueness outside of the application context, in other applications, or around the world is not required. Although true global uniqueness is impossible to guarantee without a shared knowledge scheme, a shared knowledge scheme is not required by UUID to provide uniqueness for practical implementation purposes. Implementations MAY implement a shared knowledge scheme introduced in as they see fit to extend the uniqueness guaranteed by this specification.

Unguessability Implementations SHOULD utilize a cryptographically secure pseudo-random number generator (CSPRNG) to provide values that are both difficult to predict ("unguessable") and have a low likelihood of collision ("unique"). The exception is when a suitable CSPRNG is unavailable in the execution environment. Take care to ensure the CSPRNG state is properly reseeded upon state changes, such as process forks, to ensure proper CSPRNG operation. CSPRNG ensures the best of and are present in modern UUIDs. Further advice on generating cryptographic-quality random numbers can be found in , and in .

UUIDs That Do Not Identify the Host This section describes how to generate a UUIDv1 or UUIDv6 value if an IEEE 802 address is not available, or its use is not desired. Implementations MAY leverage MAC address randomization techniques (IEEE 802.11bh) as an alternative to the pseudo-random logic provided in this section. Alternatively, implementations MAY elect to obtain a 48 bit cryptographic-quality random number as per to use it as the node ID. After generating the 48 bit fully randomized node value, implementations MUST set the least significant bit of the first octet of the node ID set to 1. This bit is the unicast/multicast bit, which will never be set in IEEE 802 addresses obtained from network cards. Hence, there can never be a conflict between UUIDs generated by machines with and without network cards. An example of generating a randomized 48 bit node value and the subsequent bit modification is detailed in the appendix. For more information about IEEE 802 address and the unicast/multicast or local/global bits please review . For compatibility with earlier specifications, note that this document uses the unicast/multicast bit, instead of the arguably more correct local/global bit because MAC addresses with the local/global bit set or not are both possible in a network. This is not the case with the unicast/multicast bit. One node cannot have a MAC address that multicasts to multiple nodes. In addition, items such as the computer's name and the name of the operating system, while not strictly speaking random, will help differentiate the results from those obtained by other systems. The exact algorithm to generate a node ID using these data is system specific, because both the data available and the functions to obtain them are often very system specific. A generic approach, however, is to accumulate as many sources as possible into a buffer, use a message digest (such as SHA-256 or SHA-512 defined by ), take an arbitrary 6 bytes from the hash value, and set the multicast bit as described above.

Sorting UUIDv6 and UUIDv7 are designed so that implementations that require sorting (e.g., database indexes) sort as opaque raw bytes, without need for parsing or introspection. Time ordered monotonic UUIDs benefit from greater database index locality because the new values are near each other in the index. As a result objects are more easily clustered together for better performance. The real-world differences in this approach of index locality vs random data inserts can be one order of magnitude or more. UUID formats created by this specification are intended to be lexicographically sortable while in the textual representation. UUIDs created by this specification are crafted with big-endian byte order (network byte order) in mind. If little-endian style is required, UUIDv8 is available for custom UUID formats.

Opacity As general guidance, it is recommend to avoid parsing UUID values unnecessarily, and instead treating UUIDs as opaquely as possible. Although application-specific concerns could of course require some degree of introspection (e.g., to examine the , or perhaps the timestamp of a UUID), the advice here is to avoid this or other parsing unless absolutely necessary. Applications typically tend to be simpler, more interoperable, and perform better, when this advice is followed.

DBMS and Database Considerations For many applications, such as databases, storing UUIDs as text is unnecessarily verbose, requiring 288 bits to represent 128 bit UUID values. Thus, where feasible, UUIDs SHOULD be stored within database applications as the underlying 128 bit binary value. For other systems, UUIDs MAY be stored in binary form or as text, as appropriate. The trade-offs to both approaches are:

Storing as binary requires less space and may result in faster data access.
Storing as text requires more space but may require less translation if the resulting text form is to be used after retrieval, which thus may make it simpler to implement.

DBMS vendors are encouraged to provide functionality to generate and store UUID formats defined by this specification for use as identifiers or left parts of identifiers such as, but not limited to, primary keys, surrogate keys for temporal databases, foreign keys included in polymorphic relationships, and keys for key-value pairs in JSON columns and key-value databases. Applications using a monolithic database may find using database-generated UUIDs (as opposed to client-generate UUIDs) provides the best UUID monotonicity. In addition to UUIDs, additional identifiers MAY be used to ensure integrity and feedback. Designers of database schema are cautioned against using name-based UUIDs (/) as primary keys in tables. A common issue observed in database schema design is the assumption that a particular value will never change, which then later turns out to be an incorrect assumption. Postal codes, license or other identification numbers, and numerous other such identifiers seem unique and unchanging at a given point time; only to later turn out to have edge cases where they need to change. The subsequent change of the identifier, used as a "name" input for name-based UUIDs, can invalidate a given database structure. In such scenarios it is observed that using any non-name-based UUID version would have resulted in the field in question being placed somewhere that would have been easier to adapt to such changes (primary key excluded from this statement). The general advice is to avoid name-based UUID natural keys and instead utilize time-based UUID surrogate keys based on the aforementioned problems detailed in this section.

IANA Considerations All references to in the IANA registries should be replaced with references to this document. References to document's Section 4.1.2 should be updated to refer to this document's . The IANA URN namespace registration for UUID filed in should be updated to reference this document. Finally IANA should track UUID Subtypes and Special Case "Namespace IDs Values" as specified in and . When evaluating requests, the designated expert(s) should consider community feedback, how well-defined is the reference specification, and this specification's requirements. Vendor-specific, application-specific, and deployment-specific values are unable to be registered. Specification documents should be published in a stable, freely available manner (ideally located with a URL) but need not be standards. The designated experts will either approve or deny the registration request, and communicate their decision to IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful.

IANA UUID Subtype Registry and Registration This specification defines the "UUID Subtype" registry for common, widely used UUID standards. should be used as-is for this registry with "This document" replaced as required. IANA UUID Subtypes

Name	ID	Subtype	Variant	Reference
Gregorian Time-based	1	version	OSF DCE / IETF	, This document
DCE Security	2	version	OSF DCE / IETF	,
MD5 Name-based	3	version	OSF DCE / IETF	, This document
Random	4	version	OSF DCE / IETF	, This document
SHA-1 Name-based	5	version	OSF DCE / IETF	, This document
Reordered Gregorian Time-based	6	version	OSF DCE / IETF	This document
Unix Time-based	7	version	OSF DCE / IETF	This document
Custom	8	version	OSF DCE / IETF	This document

This table may be extended by the "Standards Action" policy, per . For designated experts:

The minimum and maximum "ID" value for the subtype "version" within the "OSF DCE / IETF" variant is 0 through 15. The versions, within , described as "Reserved for future definition" or "unused" are omitted from this IANA registry until properly defined.
The "Subtype" column is free-form text however at the time of this publication only "version" and "family" are known UUID subtypes. The "family" subtype is part of the "Apollo NCS" variant space (Both are outside the scope of this specification). The Microsoft variant may have subtyping mechanisms defined however they are unknown and outside of the scope of this specification. Similarly, the final "Reserved for future definition" variant may introduce new subtyping logic at a future date. Subtype IDs are permitted to overlap, that is, an ID of "1" may exist in multiple variant spaces.
The "Variant" column is free-form text however it is likely one of four values will be included. The first three are "OSF DCE / IETF", "Apollo NCS", "Microsoft". The final variant value belongs to the "Reserved for future definition" variant and may introduce a new name at a future date.

IANA UUID Namespace ID Registry and Registration This specification defines the "UUID Namespace ID" registry for common, widely used Namespace ID values. The full details of this registration, including information for designated experts, can be found in .

Security Considerations Implementations SHOULD NOT assume that UUIDs are hard to guess. For example, they MUST NOT be used as security capabilities (identifiers whose mere possession grants access). Discovery of predictability in a random number source will result in a vulnerability. Implementations MUST NOT assume that it is easy to determine if a UUID has been slightly modified in order to redirect a reference to another object. Humans do not have the ability to easily check the integrity of a UUID by simply glancing at it. MAC addresses pose inherent security risks around privacy and SHOULD NOT be used within a UUID. Instead CSPRNG data SHOULD be selected from a source with sufficient entropy to ensure guaranteed uniqueness among UUID generation. See and for more information. Timestamps embedded in the UUID do pose a very small attack surface. The timestamp in conjunction with an embedded counter does signal the order of creation for a given UUID and its corresponding data but does not define anything about the data itself or the application as a whole. If UUIDs are required for use with any security operation within an application context in any shape or form then UUIDv4, SHOULD be utilized. See for MD5 Security Considerations and for SHA-1 security considerations.

Acknowledgements The authors gratefully acknowledge the contributions of Rich Salz, Michael Mealling, Ben Campbell, Ben Ramsey, Fabio Lima, Gonzalo Salgueiro, Martin Thomson, Murray S. Kucherawy, Rick van Rein, Rob Wilton, Sean Leonard, Theodore Y. Ts'o, Robert Kieffer, Sergey Prokhorenko, LiosK. As well as all of those in the IETF community and on GitHub to who contributed to the discussions which resulted in this document. This document draws heavily on the OSF DCE specification for UUIDs. Ted Ts'o provided helpful comments, especially on the byte ordering section which we mostly plagiarized from a proposed wording he supplied (all errors in that section are our responsibility, however). We are also grateful to the careful reading and bit-twiddling of Ralf S. Engelschall, John Larmouth, and Paul Thorpe. Professor Larmouth was also invaluable in achieving coordination with ISO/IEC.

References Normative References DCE: Remote Procedure Call Open Group CAE Specification C309 Information Technology, "Procedures for the operation of OSI Registration Authorities: Generation and registration of Universally Unique Identifiers (UUIDs) and their use as ASN.1 Object Identifier components" Uniform Resource Names (URNs) A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI) that is assigned under the "urn" URI scheme and a particular URN namespace, with the intent that the URN will be a persistent, location-independent resource identifier. With regard to URN syntax, this document defines the canonical syntax for URNs (in a way that is consistent with URI syntax), specifies methods for determining URN-equivalence, and discusses URI conformance. With regard to URN namespaces, this document specifies a method for defining a URN namespace and associating it with a namespace identifier, and it describes procedures for registering namespace identifiers with the Internet Assigned Numbers Authority (IANA). This document obsoletes both RFCs 2141 and 3406. Secure Hash Standard National Institute of Standards and Technology SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions National Institute of Standards and Technology DCE 1.1: Authentication and Security Services Open Group CAE Specification C311 Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References The MD5 Message-Digest Algorithm This document describes the MD5 message-digest algorithm. The algorithm takes as input a message of arbitrary length and produces as output a 128-bit "fingerprint" or "message digest" of the input. This memo provides information for the Internet community. It does not specify an Internet standard. Uniform Resource Locators (URL) This document specifies a Uniform Resource Locator (URL), the syntax and semantics of formalized information for location and access of resources via the Internet. [STANDARDS-TRACK] Randomness Requirements for Security Security systems are built on strong cryptographic algorithms that foil pattern analysis attempts. However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities. The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space. Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult. This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities. It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. A Universally Unique IDentifier (UUID) URN Namespace This specification defines a Uniform Resource Name namespace for UUIDs (Universally Unique IDentifier), also known as GUIDs (Globally Unique IDentifier). A UUID is 128 bits long, and can guarantee uniqueness across space and time. UUIDs were originally used in the Apollo Network Computing System and later in the Open Software Foundation\'s (OSF) Distributed Computing Environment (DCE), and then in Microsoft Windows platforms. This specification is derived from the DCE specification with the kind permission of the OSF (now known as The Open Group). Information from earlier versions of the DCE specification have been incorporated into this document. [STANDARDS-TRACK] Augmented BNF for Syntax Specifications: ABNF Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms This document updates the security considerations for the MD5 message digest algorithm. It also updates the security considerations for HMAC-MD5. This document is not an Internet Standards Track specification; it is published for informational purposes. Security Considerations for the SHA-0 and SHA-1 Message-Digest Algorithms This document includes security considerations for the SHA-0 and SHA-1 message digest algorithm. This document is not an Internet Standards Track specification; it is published for informational purposes. IANA Considerations and IETF Protocol and Documentation Usage for IEEE 802 Parameters Some IETF protocols make use of Ethernet frame formats and IEEE 802 parameters. This document discusses several uses of such parameters in IETF protocols, specifies IANA considerations for assignment of points under the IANA OUI (Organizationally Unique Identifier), and provides some values for use in documentation. This document obsoletes RFC 5342. Guidelines for Writing an IANA Considerations Section in RFCs Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. DNS Terminology The Domain Name System (DNS) is defined in literally dozens of different RFCs. The terminology used by implementers and developers of DNS protocols, and by operators of DNS systems, has sometimes changed in the decades since the DNS was first defined. This document gives current definitions for many of the terms used in the DNS in a single document. This document obsoletes RFC 7719 and updates RFC 2308. Randomness Improvements for Security Protocols Randomness is a crucial ingredient for Transport Layer Security (TLS) and related security protocols. Weak or predictable "cryptographically secure" pseudorandom number generators (CSPRNGs) can be abused or exploited for malicious purposes. An initial entropy source that seeds a CSPRNG might be weak or broken as well, which can also lead to critical and systemic security problems. This document describes a way for security protocol implementations to augment their CSPRNGs using long-term private keys. This improves randomness from broken or otherwise subverted CSPRNGs. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. Information technology – Open Systems Interconnection – The Directory: Overview of concepts, models and services Information technology – Procedures for the operation of object identifier registration authorities: General procedures and top arcs of the international object identifier tree Information Technology - Abstract Syntax Notation One (ASN.1) & ASN.1 encoding rules A Scala client for Cassandra Twitter Snowflake is a network service for generating unique ID numbers at high scale with some simple guarantees. Twitter Flake: A decentralized, k-ordered id generation service in Erlang Boundary Sharding & IDs at Instagram Instagram Engineering K-Sortable Globally Unique IDs Segment Sequential UUID / Flake ID generator pulled out of elasticsearch common Flake ID Generator A distributed unique ID generator inspired by Twitter's Snowflake Sony Laravel: The mysterious "Ordered UUID" Creating sequential GUIDs in C# for MSSQL or PostgreSql Universally Unique Lexicographically Sortable Identifier sid : generate sortable identifiers The 2^120 Ways to Ensure Unique Identifiers Google Globally Unique ID Generator ObjectId - MongoDB Manual MongoDB Collision-resistant ids optimized for horizontal scaling and performance. IEEE Standard for Floating-Point Arithmetic. IEEE Uniform Resource Names (URN) Namespaces IANA UUID objects according to RFC Python curly braced GUID string Microsoft Why does COM express GUIDs in a mix of big-endian and little-endian? Why can’t it just pick a side and stick with it? uuid_gen Command (NCS) IBM Random Number Generator Recommendations for Applications

Test Vectors Both UUIDv1 and UUIDv6 test vectors utilize the same 60 bit timestamp: 0x1EC9414C232AB00 (138648505420000000) Tuesday, February 22, 2022 2:22:22.000000 PM GMT-05:00 Both UUIDv1 and UUIDv6 utilize the same values in clock_seq, and node. All of which have been generated with random data. For the randomized node, the least significant bit of the first octet is set to a value of 1 as per . Thus the starting value 0x9E6BDECED846 was changed to 0x9F6BDECED846. The pseudocode used for converting from a 64 bit Unix timestamp to a 100ns Gregorian timestamp value has been left in the document for reference purposes.

Test Vector Timestamp Pseudo-code

Example of a UUIDv1 Value

UUIDv1 Example Test Vector

Example of a UUIDv3 Value The MD5 computation from is detailed in using the DNS Namespace ID value and the Name "www.example.com". while the field mapping and all values are illustrated in . Finally to further illustrate the bit swapping for version and variant see .

UUIDv3 Example MD5

UUIDv3 Example Test Vector

UUIDv3 Example Ver Var bit swaps

Example of a UUIDv4 Value This UUIDv4 example was created by generating 16 bytes of random data resulting in the hexadecimal value of 919108F752D133205BACF847DB4148A8. This is then used to fill out the fields as shown in . Finally to further illustrate the bit swapping for version and variant see .

UUIDv4 Example Test Vector

UUIDv4 Example Ver/Var bit swaps

Example of a UUIDv5 Value The SHA-1 computation from is detailed in using the DNS Namespace ID value and the Name "www.example.com". while the field mapping and all values are illustrated in . Finally to further illustrate the bit swapping for version and variant and the unused/discarded part of the SHA-1 value see .

UUIDv5 Example SHA-1

UUIDv5 Example Test Vector

UUIDv5 Example Ver/Var bit swaps and discarded SHA-1 segment

Example of a UUIDv6 Value

UUIDv6 Example Test Vector

Example of a UUIDv7 Value This example UUIDv7 test vector utilizes a well-known Unix epoch timestamp with millisecond precision to fill the first 48 bits. rand_a and rand_b are filled with random data. The timestamp is Tuesday, February 22, 2022 2:22:22.00 PM GMT-05:00 represented as 0x017F22E279B0 or 1645557742000

UUIDv7 Example Test Vector

Illustrative Examples The following sections contain illustrative examples which serve to show how one may use UUIDv8 for custom and/or experimental application based logic. The examples below have not been through the same rigorous testing, prototyping, and feedback loop that other algorithms in this document have undergone. The authors encouraged implementors to create your own UUIDv8 algorithm rather than use the items defined in this section.

Example of a UUIDv8 Value (time-based) This example UUIDv8 test vector utilizes a well-known 64 bit Unix epoch timestamp with 10ns precision, truncated to the least-significant, right-most, bits to fill the first 60 bits of custom_a and custom_b while setting the version bits between these two segments to the version value of 8. The variant bits are set and the final segment, custom_c, is filled with random data. Timestamp is Tuesday, February 22, 2022 2:22:22.000000 PM GMT-05:00 represented as 0x2489E9AD2EE2E00 or 164555774200000000 (10ns-steps).

UUIDv8 Example Time-based Illustrative Example

Example of a UUIDv8 Value (name-based) As per name-based UUIDs that desire to use modern hashing algorithms MUST be created within the UUIDv8 space. These MAY leverage newer hashing algorithms such as SHA-256 or SHA-512 defined by , SHA-3 or SHAKE defined by , or even algorithms that have not been defined yet. A SHA-256 version of is detailed in as an illustrative example detailing how this can be achieved. The creation of the name-based UUIDv8 value in this section follows the same logic defined in with the difference being SHA-256 in place of SHA-1. The field mapping and all values are illustrated in . Finally to further illustrate the bit swapping for version and variant and the unused/discarded part of the SHA-256 value see . An important note for secure hashing algorithms that produce outputs of an arbitrary size, such as those found in SHAKE, the output hash MUST be 128 bits or larger.

UUIDv8 Example SHA256

UUIDv8 Example Name-Based SHA-256 Illustrative Example

UUIDv8 Example Ver/Var bit swaps and discarded SHA-256 segment Examining :

Line A details the full SHA-256 as a hexadecimal value with the dashes inserted.
Line B details the version and variant hexadecimal positions which must be overwritten.
Line C details the final value after the ver/var have been overwritten.
Line D details the discarded, leftover values from the original SHA-256 computation.