UUID Variant

Definition

The variant is a bit field within a UUID that identifies which layout specification the UUID conforms to. It tells parsing software which rules to apply when reading the UUID’s internal structure.

RFC 9562 defines the variant as occupying the two most significant bits of byte 8 (the first byte of the fourth group in the string representation). For standard RFC 9562 UUIDs, these two bits are always 10 (binary).

Where to find it

In the canonical UUID string format, the variant appears as the first character of the fourth group — position 17 when counting from 1:

550e8400-e29b-41d4-a716-446655440000
                   ^
                   position 17 — the variant digit

For any RFC 9562-compliant UUID, this character will always be one of: 8, 9, a, or b.

Why four possible values?

The variant field uses the top two bits of the byte, not the whole byte. Setting those two bits to 10 leaves 6 remaining bits free for other data — those 6 bits form part of the UUID’s payload (random data, timestamp bits, etc.).

The four hex digits that start with binary 10 are:

All four are valid for RFC 9562 UUIDs. The remaining two bits (00, 01, 10, 11) within those nibbles are part of the payload data, not the variant.

Variant values defined by RFC 9562

RFC 9562 defines three variant values:

The 10xxxxxx variant is what you will encounter in virtually all modern UUIDs. The NCS and Microsoft variants exist only in legacy systems predating the RFC.

Variant vs Version

These two fields are often confused:

Example — identifying both fields at a glance:

550e8400-e29b-41d4-a716-446655440000
              ^    ^
              |    variant: 'a' → RFC 9562 (bits 10xxxxxx)
              version: '4' → randomly generated (UUID v4)

Practical use

In most application code, you never need to inspect or set the variant. UUID libraries handle it automatically. You may encounter it when:

• Validating a UUID string — checking that position 17 is 8, 9, a, or b confirms it is an RFC 9562 UUID
• Debugging legacy data — a UUID with a 0 or c–f at position 17 is from a pre-RFC system
• Implementing a UUID parser from scratch

Related terms

• UUID — the full identifier that contains the variant field
• Namespace — a UUID used as a scope for name-based generation
• Entropy — where the non-structural bits in a UUID come from