UUID Version 3 (name-based, MD5)

UUIDv3 is a name-based UUID derived deterministically by MD5-hashing a namespace UUID concatenated with a name, then overwriting the version (3) and variant bits. The same namespace + name always yields the same UUID, so it is used to mint stable ids from URLs, DNS names, or OIDs. UUIDv5 is the SHA-1 equivalent and is preferred for new designs.

identifier

spec: RFC 9562 §5.3

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

example:

5df41881-3aed-3515-88a7-2f4a814cf09e

field	bits	meaning
md5_high	48	High 48 bits of MD5(namespace_uuid \|\| name).
ver	4	version = 0b0011 (3), overwriting hash bits.
md5_mid	12	Next 12 bits of the MD5 digest.
var	2	variant = 0b10, overwriting hash bits.
md5_low	62	Remaining 62 bits of the MD5 digest.

Deterministic: UUIDv3(namespace, name) = set_version_variant( MD5( bytes(namespace) || bytes(name) ), 3 ). Same inputs -> same UUID.
Example uses the DNS namespace (6ba7b810-9dad-11d1-80b4-00c04fd430c8) with name 'www.example.com', yielding 5df41881-3aed-3515-88a7-2f4a814cf09e (RFC 9562 Appendix).
MD5 is used only for namespacing, not security; prefer UUIDv5 (SHA-1) for new applications per RFC 9562.

agent: curl -H 'accept: application/json' wire.phall.io/identifier/uuidv3 or /identifier/uuidv3.json

UUID Version 3 (name-based, MD5)

identifier

provenance

see also