Can I have "private" UUIDs

Question

Can I cleanly use a private UUID variant/version?

I use random UUIDs which I essentially consider to be big integers. Now I have the case where I would like to generate a "private" UUID which is not based on any one of the well known 5 variants/versions.

Should I "hijack" a well known variant/version I'll never ever use? Or should I use an unknown variant/version?

Neither the RFC nor wikipedia covers this topic.

Answer 1

Nobody answered so I'll offer my considerations.

We use random UUIDs to identify objects within clear system boundaries. Although UUIDs are represented by 128 bits, there can only be 2 ^ 121 random UUIDs because the version (4 bits) and the variant (3 bits) are constants. (Actually variant 4 allows to use one additional bit and for the pedantic this could be added to the 121.)

I know we will never use versions 1, 2, 3 and 5. Unfortunately there are no provisions mentioned for the remaining 11 (16 - 5) version numbers in the RFC.

What I resolved to is to claim UUIDs of version 1 and variant 0100 as private (or local) ones. I reserved 4 bits for my own sub-type which leaves me an ample 117 bits per sub-type for my own use. For a few subsystems with deterministic IDs I can now create UUIDs that fit in the structures I use.

I am fully aware that these IDs might theoretically clash with externally generated UUIDs. But as the system boundaries are clear and external IDs are considered separately, this approach suits us and is practicable.