Reference
UUID Reference
What each UUID version is made of, when to choose it and what the bits guarantee — with a generator one click away.
One name, several constructions
A UUID is always 128 bits. The version field says how those bits were produced: random (v4), time based (v1, v7), name based (v3, v5) or vendor defined (v2). The version does not change the storage size, only the guarantees you get about ordering and uniqueness.
Identifiers are not secrets
A UUID is designed to be unique, not unguessable. v1 embeds a timestamp and often a MAC address; v7 embeds a millisecond timestamp. If an identifier must not be enumerable, treat it as public data and authorize the request, or use a separate random token.
All entries
UUID format128 bits written as 32 hex digits in five hyphenated groups.
UUID v4122 random bits — the default choice for identifiers that need no ordering.
UUID v7A 48-bit Unix millisecond timestamp followed by random bits, for sortable identifiers.
UUID v1Timestamp plus clock sequence plus node, historically a MAC address.
UUID v3 and v5Deterministic identifiers derived from a namespace and a name via MD5 or SHA-1.
Nil UUID00000000-0000-0000-0000-000000000000, the all-zero UUID.
Max UUIDffffffff-ffff-ffff-ffff-ffffffffffff, the all-one UUID.
UUID version and variant bitsThe fixed bits that tell a parser how the identifier was built.
UUID collision riskWhy 122 random bits is safe at scale, and when name-based IDs collide deliberately.
UUID vs ULIDTwo 128-bit identifiers with different text encodings and ordering guarantees.
GUID vs UUIDMicrosoft's GUID is a UUID with platform-specific formatting and byte order.