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Asked: 2026-05-26T05:40:33+00:00

The UUID specification defines 4 predefined namespaces which it describes as "potentially interesting" –

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The UUID specification defines 4 predefined namespaces which it describes as "potentially interesting" – meaning among other things, "if other people have generated UUIDs in this namespace you can verify them":

  • 6ba7b810-9dad-11d1-80b4-00c04fd430c8 for DNS
  • 6ba7b811-9dad-11d1-80b4-00c04fd430c8 for URL
  • 6ba7b812-9dad-11d1-80b4-00c04fd430c8 for ISO OID
  • 6ba7b814-9dad-11d1-80b4-00c04fd430c8 for X.500 DN

Where did these come from?

Specifically;

  • If I’m generating my own namespace UUID do I need to avoid anything in particular?
  • I’m aware how big the UUID space is, but does this have any implication on collisions?
  • Why have they chosen the 4th octet to increase as a kind of UUID ‘version number’?
  • Do my questions imply that I’m missing something fundamental about UUIDs?
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1 Answer

  1. Editorial Team

    First, to be clear, this whole discussion is limited to version 3 & 5 UUIDs. In my (anecdotal) experience, version 4 (random) UUIDs are most commonly used.

    4122‘s namespaced UUID generation algorithm ambiguously begins:

    Allocate a UUID to use as a "name space ID"

    There is no other mention of "name space ID" allocation, and neither I nor python have found any standardized spaces beyond the four listed in RFC 4122.

    So the answer to your first question,

    • If I’m generating my own namespace UUID do I need to avoid anything in particular?

    You only need to avoid the four standard namespaces.

    The next question,

    • I’m aware how big the UUID space is, but does this have any implication on collisions?

    Has two parts:

    1. Will UUIDs within your namespace collide? Verbatim from 4122:

      The UUIDs generated from two different names in [your] namespace should be different (with very high probability).

    2. Will your namespace UUID collide with other namespaces? I couldn’t find a direct answer, since there’s no standard for "name space ID" allocation, but the argument in section 4.1.1 seems relevant:

      Interoperability, in any form, with variants other than the one
      defined here is not guaranteed, and is not likely to be an issue in
      practice.

    • Why have they chosen the 4th octet to increase as a kind of UUID ‘version number’?

    This one’s a bit of a mystery. Luckily, we have a spec for UUIDs, so we can mine them for some insight.

    Note that the (0-index) 8th octet starts with 8 in all cases, so we’re dealing with RFC 4122 variant UUIDs. Phew.

    Now check octet 6 for the version: 1, we’re dealing with version 1 time-based UUIDs.

    This answer has a handy algorithm for extracting python datetimes from version 1 UUIDs. Applying the algorithm yields a time in February 4th, 1998. I have yet to find meaning in this date. Incrementing the 3rd octet adds the smallest encodable time interval (100ns) to the date.

    • Do my questions imply that I’m missing something fundamental about UUIDs?

    Nope. There is very little discussion of UUID namespaces, since random UUIDs are so easy.

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