5.4. UUID Version 4

UUIDv4 is meant for generating UUIDs from truly random or pseudorandom numbers.

An implementation may generate 128 bits of random data that is used to fill out the UUID fields in Figure 8. The UUID version and variant then replace the respective bits as defined by Sections 4.1 and 4.2.

Alternatively, an implementation MAY choose to randomly generate the exact required number of bits for random_a, random_b, and random_c (122 bits total) and then concatenate the version and variant in the required position.

For guidelines on random data generation, see Section 6.9.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           random_a                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          random_a             |  ver  |       random_b        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var|                       random_c                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           random_c                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 8: UUIDv4 Field and Bit Layout

Field Definitions:

random_a: The first 48 bits of the layout that can be filled with random data as specified in Section 6.9. Occupies bits 0 through 47 (octets 0-5).

ver: The 4-bit version field as defined by Section 4.2, set to 0b0100 (4). Occupies bits 48 through 51 of octet 6.

random_b: 12 more bits of the layout that can be filled random data as per Section 6.9. Occupies bits 52 through 63 (octets 6-7).

var: The 2-bit variant field as defined by Section 4.1, set to 0b10. Occupies bits 64 and 65 of octet 8.

random_c: The final 62 bits of the layout immediately following the var field to be filled with random data as per Section 6.9. Occupies bits 66 through 127 (octets 8-15).

5.4. UUID Version 4​

5.4. UUID Version 4