128-bit binary value. It can be used as a more convenient container for 16-byte binary strings, which will be pretty printed as UUID or IPv6 address. => https://datatracker.ietf.org/doc/html/rfc9562 UUID Application is left responsible for UUID validation. Simplest decoder can safely replace HEXLET's tag with 0x90 and decode it as ordinary 16-byte binary string. Example representations: Nil UUID | 04 00000000000000000000000000000000 Max UUID | 04 FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF 0e875e3f-d385-49eb-87b4-be42d641c367 | 04 0E875E3FD38549EB87B4BE42D641C367 2001:db8::1234 IPv6 | 04 20010db8000000000000000000001234