UUID Version 4 (random)

UUIDv4 is a 128-bit identifier that is random except for the 6 fixed bits: a 4-bit version (0b0100) and a 2-bit variant (0b10). The remaining 122 bits come from a cryptographically strong RNG, giving ~5.3e36 distinct values. It is the default UUID for most libraries because it needs no clock, MAC address, or coordination, at the cost of not being time-sortable.

identifier

spec: RFC 9562 §5.4

length 128 bits not sortable hex with dashes (8-4-4-4-12)

example:

919108f7-52d1-4320-9bac-f847db4148a8

field	bits	meaning
rand_a	48	Random bits.
ver	4	version = 0b0100 (4).
rand_b	12	Random bits.
var	2	variant = 0b10 (RFC 9562).
rand_c	62	Random bits.

122 of the 128 bits are random; only the version (4 bits) and variant (2 bits) are fixed. The example's 13th hex char is '4' and the variant nibble begins '10' (here '9').
Not time-sortable and not deterministic; for database primary keys that benefit from index locality, prefer UUIDv7.
Random generation MUST use a cryptographically secure RNG (RFC 9562); Math.random()-style sources are non-conformant.

agent: curl -H 'accept: application/json' wire.phall.io/identifier/uuidv4 or /identifier/uuidv4.json

UUID Version 4 (random)

identifier

provenance

see also