A 128-bit label that uses a combination of locally specific information (such as Medium access control (MAC) address) and local date-time to create an essentially unique identifier. [Per Wikipedia](https://en.wikipedia.org/wiki/Universally_unique_identifier#:~:text=A%20Universally%20Unique%20Identifier%20(UUID,Acronym), there have been multiple standards. Note: although they are called “versions,” they do not represent a progression of iterative improvements; rather, they are just numbered variants.
| Version | Basis |
|---|---|
| v1 | date-time + MAC address |
| v2 | date-time + MAC address |
| v3 | Some kind of namespace-based system |
| v4 | Just totally random |
| v5 | Some kind of namespace-based system |
| v6 | date-time + MAC address |
| v7 | timestamp + counter + random number |
| v8 | open-ended, system-specific |