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What is a GUID?

GUID or UUID is an acronym for Globally Unique Identifier or Universally Unique Identifier. It is a 128-bit integer number used to identify resources. The term globally unique identifier or GUID is also used, often in software created by Microsoft, where as other technologies use Universally Unique Identifier or UUID.

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.

How Unique is a GUID or UUID?

Anyone can create a GUID or UUID and use it to identify something with near certainty that the identifier does not duplicate one that has already been, or will be, created to identify something else. Information labeled with UUIDs by independent parties can therefore be later combined into a single database or transmitted on the same channel, with a negligible probability of duplication.

GUID and UUID 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-a456-426614174000
  • xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
The four-bit M and the 1 to 3 bit N fields code the format of the UUID itself.

The four bits of digit M are the UUID version, and the 1 to 3 most significant bits of digit N code the UUID variant. (See below.) In the example, M is 1, and N is a (10xx2), meaning that this is a version-1, variant-1 UUID; that is, a time-based DCE/RFC 4122 UUID.

Nil or Empty GUID

The "nil" UUID, a special case, is the UUID 00000000-0000-0000-0000-000000000000; that is, all bits set to zero.

GUID / UUID Latest Version

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.