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Universally unique identifier
A universally unique identifier (UUID) is a 128-bit label used for information in computer systems. The term globally unique identifier (GUID) is also used.
When generated according to the standard methods, UUIDs are, for practical purposes, unique. Their uniqueness does not depend on a central registration authority or coordination between the parties generating them, unlike most other numbering schemes. While the probability that a UUID will be duplicated is not zero, it is close enough to zero to be negligible.
Versions
For both variants 1 and 2, five "versions" are defined in the standards, and each version may be more appropriate than the others in specific use cases. Version is indicated by the M in the string representation.
Version-1 UUIDs are generated from a time and a node ID (usually the MAC address); version-2 UUIDs are generated from an identifier (usually a group or user ID), time, and a node ID; versions 3 and 5 produce deterministic UUIDs generated by hashing a namespace identifier and name; and version-4 UUIDs are generated using a random or pseudo-random number.
Nil UUID
The "nil" UUID, a special case, is the UUID 00000000-0000-0000-0000-000000000000; that is, all bits set to zero.
Version 4 (random)
A version 4 UUID is randomly generated. As in other UUIDs, 4 bits are used to indicate version 4, and 2 or 3 bits to indicate the variant (102 or 1102 for variants 1 and 2 respectively). Thus, for variant 1 (that is, most UUIDs) a random version-4 UUID will have 6 predetermined variant and version bits, leaving 122 bits for the randomly generated part, for a total of 2122, or 5.3×1036 (5.3 undecillion) possible version-4 variant-1 UUIDs. There are half as many possible version-4 variant-2 UUIDs (legacy GUIDs) because there is one fewer random bit available, 3 bits being consumed for the variant.
Format
In its canonical textual representation, the 16 octets of a UUID are represented as 32 hexadecimal (base-16) digits, displayed in five groups separated by hyphens, in the form 8-4-4-4-12 for a total of 36 characters (32 hexadecimal characters and 4 hyphens). For example:
123e4567-e89b-12d3-a456-426614174000xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
UUID record layout
| Name | Length (bytes) | Length (hex digits) | Length (bits) | Contents |
|---|---|---|---|---|
| time_low | 4 | 8 | 32 | integer giving the low 32 bits of the time |
| time_mid | 2 | 4 | 16 | integer giving the middle 16 bits of the time |
| time_hi_and_version | 2 | 4 | 16 | 4-bit "version" in the most significant bits, followed by the high 12 bits of the time |
| clock_seq_hi_and_res clock_seq_low | 2 | 4 | 16 | 1 to 3-bit "variant" in the most significant bits, followed by the 13 to 15-bit clock sequence |
| node | 6 | 12 | 48 | the 48-bit node id |