Editing Universally unique identifier (section)

==== 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 (10<sub>2</sub> or 110<sub>2</sub> 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 2<sup>122</sup>, or 5.3{{e|36}} (5.3&nbsp;[[Names of large numbers|undecillion]]) possible version-4 variant-1 UUIDs.  There are half as many possible version 4, variant 2 UUIDs (legacy GUIDs) because there is one less random bit available, 3 bits being consumed for the variant.

Per RFC 9562{{r|RFC 9562}}, the seventh octet's most significant 4 bits indicate which version the UUID adheres to. This means that the first hexadecimal digit in the third group always starts with a <code>4</code> in UUIDv4s. Visually, this looks like this <code>xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx</code>, where <code>M</code> is the UUID version field. The upper two or three bits of digit <code>N</code> encode the variant. Values are <code>8</code>, <code>9</code>, <code>A</code> or <code>B</code> for the 2 bit indication, values <code>C</code> or <code>D</code> for the 3 bit indication. For example, a random UUID version 4, variant 1 could be  <code>8D8AC610-566D-4EF0-9C22-186B2A5ED793</code>.<ref>{{Cite web |date=2023-11-06 |title=draft-ietf-uuidrev-rfc4122bis-14 |url=https://datatracker.ietf.org/doc/html/draft-ietf-uuidrev-rfc4122bis#name-version-field |url-status=live |archive-url=https://archive.today/20240417032603/https://datatracker.ietf.org/doc/html/draft-ietf-uuidrev-rfc4122bis#selection-6225.0-6239.184 |archive-date=2024-04-17 |website=[[University of Washington]]}}</ref>