Editing Universally unique identifier (section)

==== Version 2 (date-time and MAC address, DCE security version) ====

RFC 9562{{r|RFC 9562}} reserves version 2 for "DCE security" UUIDs; but it does not provide any details. For this reason, many UUID implementations omit version 2. However, the specification of version-2 UUIDs is provided by the DCE 1.1 Authentication and Security Services specification.<ref name="dce_spec_auth" />

Version-2 UUIDs are similar to version 1, except that the least significant 8 bits of the clock sequence are replaced by a "local domain" number, and the least significant 32 bits of the timestamp are replaced by an integer identifier meaningful within the specified local domain. On [[POSIX]] systems, local-domain numbers 0 and 1 are for user ids ([[User identifier|UIDs]]) and group ids ([[Group identifier|GIDs]]) respectively, and other local-domain numbers are site-defined.<ref name="dce_spec_auth" /> On non-POSIX systems, all local domain numbers are site-defined.

The ability to include a 40-bit domain/identifier in the UUID comes with a tradeoff. On the one hand, 40 bits allow about 1&nbsp;trillion domain/identifier values per node ID.  On the other hand, with the clock value truncated to the 28 most significant bits, compared to 60 bits in version 1, the clock in a version 2 UUID will "tick" only once every 429.49&nbsp;seconds, a little more than 7&nbsp;minutes, as opposed to every 100&nbsp;nanoseconds for version 1.  And with a clock sequence of only 6&nbsp;bits, compared to 14 bits in version 1, only 64 unique UUIDs per node/domain/identifier can be generated per 7-minute clock tick, compared to 16,384 clock sequence values for version 1.<ref>{{cite web |last1=Kuchling |first1=A. M. |title=What's New in Python 2.5 |url=https://docs.python.org/3/whatsnew/2.5.html |access-date=23 January 2016 |website=Python.org |ref=Python2.5RelNotes}}</ref>