In more detail
A UUID is 32 Hexadecimal digits in groups of 8-4-4-4-12, such as 123e4567-e89b-12d3-a456-426614174000. The first digit of the third group gives the version, and the first digit of the fourth group (8, 9, a or b for standard UUIDs) gives the variant. Version 4 UUIDs are random (122 random bits), so two systems can create identifiers independently with a negligible chance of clashing. Version 7, standardised in RFC 9562 (2024), starts with a 48-bit millisecond timestamp so UUIDs sort by creation time, which helps database indexes.
Why it matters
UUIDs let distributed systems, offline apps and merged databases create IDs without coordination. Random version 4 keys scatter inserts across an index, which can slow large databases; version 7 avoids that but reveals when each record was created. UUIDs are identifiers, not secrets: they should not be used as passwords, session tokens or unguessable links.
Example
In f47ac10b-58cc-4372-a567-0e02b2c3d479 the 4 at the start of the third group marks version 4 and the a at the start of the fourth group the standard variant. You would need about 2.7 × 10¹⁸ random version 4 UUIDs for a 50% chance of a single duplicate. A version 7 UUID created at 12:00 UTC on 9 October 2026 begins 01a12088-d200-7, the hex of 1,791,547,200,000 milliseconds.