Version 4: 122 of the 128 bits are random, the other 6 carry the version and variant — which is why the 13th hex digit is always 4 and the 17th is always 8, 9, a or b. These UUIDs come from a seeded, non-cryptographic PRNG, so the same seed always returns the same list — that is what makes a shared link reproducible, and exactly why they must never be used as real identifiers. For production, call crypto.randomUUID().
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10edcd13-afe9-4758-891b-8df71747f52d
Version 4: 122 of the 128 bits are random, the other 6 carry the version and variant — which is why the 13th hex digit is always 4 and the 17th is always 8, 9, a or b. These UUIDs come from a seeded, non-cryptographic PRNG, so the same seed always returns the same list — that is what makes a shared link reproducible, and exactly why they must never be used as real identifiers. For production, call crypto.randomUUID().
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Về uuid
The format predates the web. In the early 1980s Apollo Computer needed identifiers that any machine on a network could mint without asking a central authority, and built 64-bit unique IDs into its Domain/OS. The idea moved into Apollo's Network Computing System, widened to 128 bits, and from there into the Open Software Foundation's Distributed Computing Environment RPC specification at the start of the 1990s. Microsoft adopted the DCE design for COM and called the result a GUID — the same sixteen bytes, but historically printed with the first three fields byte-swapped, which is why one identifier can appear in two different spellings.
Paul Leach, Michael Mealling and Rich Salz finally wrote the format down for the IETF in July 2005 as RFC 4122, which registered the urn:uuid namespace and defined five versions: time-and-MAC (1), DCE Security (2), MD5 name-based (3), random (4) and SHA-1 name-based (5). ISO and the ITU standardised the same object as ISO/IEC 9834-8 and ITU-T X.667. Version 1's embedded MAC address turned out to be a privacy leak — GUIDs written into Microsoft Office documents carried the author's network card address, and this is widely reported to have helped investigators identify the author of the 1999 Melissa macro virus, though the accounts are second-hand.
Version 4 won on simplicity and then lost on database performance: random keys scatter inserts across a B-tree index instead of landing at its right-hand edge. Developers built time-ordered alternatives of their own — MongoDB's ObjectId in 2009, Twitter's Snowflake in 2010, Alizain Feerasta's ULID in 2016. A draft by Brad Peabody and Kyzer Davis folded that lesson back into the standard, and in May 2024 RFC 9562 replaced RFC 4122, adding version 6, version 7 (48 bits of Unix milliseconds followed by randomness, so the lowercase text form sorts by creation time) and version 8 for custom layouts.
Tính chất chính
The canonical text form is 36 characters: 32 hexadecimal digits in groups of 8-4-4-4-12 separated by four hyphens.
A version 4 UUID carries 122 random bits — 128 minus 4 version bits and 2 variant bits — giving 2^122 ≈ 5.3 × 10^36 possible values.
The version is the first hex digit of the third group; the variant occupies the top bits of the first hex digit of the fourth group, which for RFC 4122 and RFC 9562 UUIDs is 8, 9, a or b.
Drawing 103 trillion random version 4 UUIDs gives roughly a one-in-a-billion chance that two of them are identical, by the birthday approximation n²/2N.
A version 7 UUID begins with a 48-bit big-endian count of milliseconds since the Unix epoch, so sorting the lowercase text form sorts by creation time.
The nil UUID is all 128 bits zero; RFC 9562 also defines a Max UUID with all 128 bits set.
RFC 9562 (May 2024) obsoletes RFC 4122 (July 2005) and adds versions 6, 7 and 8 to the original five.
The UUIDs on this page are produced by a seeded, non-cryptographic PRNG and are reproducible by anyone with the link, so they are deliberately unsuitable as real identifiers.