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.. _rfc4122:

==============
RFC 4122 UUIDs
==============

.. toctree::
    :titlesonly:
    :hidden:

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

`RFC 4122`_ defines five versions of UUID, while a `new Internet-Draft under
review`_ defines three new versions. 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 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: Custom
    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://tools.ietf.org/html/rfc4122
.. _new Internet-Draft under review: https://datatracker.ietf.org/doc/html/draft-ietf-uuidrev-rfc4122bis-00