UUID Format Explained (8-4-4-4-12)

A UUID (Universally Unique Identifier) is a 128-bit value typically represented as 32 hexadecimal digits separated by four hyphens in the pattern 8-4-4-4-12: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx.

Anatomy of the string format:

550e8400-e29b-41d4-a716-446655440000
|      | |  | |  | |  | |          |
|  8   | | 4| | 4| | 4| |    12    |
|      | |  | |  | |  | |          |
time_lo  mid  ver  var   node/random
              +hi  +seq

• Group 1 (8 hex = 32 bits): In v1, this is time_low. In v7, it is the high 32 bits of the timestamp. In v4, it is random.
• Group 2 (4 hex = 16 bits): In v1, this is time_mid. In v7, it is the low 16 bits of the timestamp. In v4, it is random.
• Group 3 (4 hex = 16 bits): The first digit (M) is the version number. The remaining 12 bits vary by version.
• Group 4 (4 hex = 16 bits): The first digit (N) encodes the variant in its high bits. For standard UUIDs, the first hex digit is 8, 9, a, or b (binary starts with 10). The remaining bits are clock sequence or random.
• Group 5 (12 hex = 48 bits): In v1, this is the node ID (MAC address). In v4 and v7, it is random data.

How to read the version: Look at the first character of the third group. A 4 means v4, a 7 means v7, a 1 means v1, and so on.

How to read the variant: Look at the first character of the fourth group. If it is 8, 9, a, or b, the UUID follows the RFC 4122/9562 variant. Values 0-7 indicate the legacy NCS variant, and c-f indicate the legacy Microsoft variant.

Case sensitivity: UUIDs are case-insensitive per the RFC. Both 550E8400-E29B-41D4-A716-446655440000 and 550e8400-e29b-41d4-a716-446655440000 represent the same UUID. Most implementations output lowercase, and RFC 9562 recommends lowercase for new implementations.

Alternative representations:

• Without hyphens: 550e8400e29b41d4a716446655440000 (32 chars)
• URN format: urn:uuid:550e8400-e29b-41d4-a716-446655440000
• Binary: 16 raw bytes (most compact, used in databases)
• Base64: 22 characters (e.g., VQ6EAOKbQdSnFkRmVUQAAA==)