.. _rfc4122:

==================================
RFC 9562 (formerly RFC 4122) UUIDs
==================================

.. toctree::
    :titlesonly:
    :hidden:

    rfc4122/version1
    rfc4122/version2
    rfc4122/version3
    rfc4122/version4
    rfc4122/version5
    rfc4122/version6
    rfc4122/version7
    rfc4122/version8

`RFC 9562`_ (formerly `RFC 4122`_) defines eight versions of UUIDs. Each version has different generation algorithms and
properties. Which one you choose depends on your use-case. You can find out more about their applications on the
specific page for that version.

Version 1: Gregorian Time
    This version of UUID combines a timestamp, node value (in the form of a MAC address from the local computer's
    network interface), and a clock sequence to ensure uniqueness. For more details, see :doc:`rfc4122/version1`.

Version 2: DCE Security
    This version of UUID is the same as Version 1, except the ``clock_seq_low`` field is replaced with a *local domain*
    and the ``time_low`` field is replaced with a *local identifier*. For more details, see :doc:`rfc4122/version2`.

Version 3: Name-based (MD5)
    This version of UUID hashes together a namespace and a name to create a deterministic UUID. The hashing algorithm
    used is MD5. For more details, see :doc:`rfc4122/version3`.

Version 4: Random
    This version creates a UUID using truly-random or pseudo-random numbers. For more details, see :doc:`rfc4122/version4`.

Version 5: Named-based (SHA-1)
    This version of UUID hashes together a namespace and a name to create a deterministic UUID. The hashing algorithm
    used is SHA-1. For more details, see :doc:`rfc4122/version5`.

Version 6: Reordered Gregorian Time
    This version of UUID combines the features of a :ref:`version 1 UUID <rfc4122.version1>` with a *monotonically
    increasing* UUID. For more details, see :ref:`rfc4122.version6`.

Version 7: Unix Epoch Time
    This version of UUID combines a timestamp--based on milliseconds elapsed since the Unix Epoch--and random bytes to
    create a monotonically increasing, sortable UUID without the privacy and entropy concerns associated with version 1
    and version 6 UUIDs. For more details, see :ref:`rfc4122.version7`.

Version 8: Implementation-Specific, Custom Format
    This version of UUID allows applications to generate custom identifiers in an RFC-compatible format. For more
    details, see :doc:`rfc4122/version8`.

.. _RFC 4122: https://www.rfc-editor.org/rfc/rfc4122
.. _RFC 9562: https://www.rfc-editor.org/rfc/rfc9562