5.1. UUID Version 1

UUIDv1 is a time-based UUID featuring a 60-bit timestamp represented by Coordinated Universal Time (UTC) as a count of 100-nanosecond intervals since 00:00:00.00, 15 October 1582 (the date of Gregorian reform to the Christian calendar).

UUIDv1 also features a clock sequence field that is used to help avoid duplicates that could arise when the clock is set backwards in time or if the Node ID changes.

The node field consists of an IEEE 802 MAC address, usually the host address or a randomly derived value per Sections 6.9 and 6.10.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           time_low                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           time_mid            |  ver  |       time_high       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var|         clock_seq         |             node              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                              node                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 6: UUIDv1 Field and Bit Layout

Field Definitions:

time_low: The least significant 32 bits of the 60-bit starting timestamp. Occupies bits 0 through 31 (octets 0-3).

time_mid: The middle 16 bits of the 60-bit starting timestamp. Occupies bits 32 through 47 (octets 4-5).

ver: The 4-bit version field as defined by Section 4.2, set to 0b0001 (1). Occupies bits 48 through 51 of octet 6.

time_high: The least significant 12 bits from the 60-bit starting timestamp. 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.

clock_seq: The 14 bits containing the clock sequence. Occupies bits 66 through 79 (octets 8-9).

node: 48-bit spatially unique identifier. Occupies bits 80 through 127 (octets 10-15).

For systems that do not have UTC available but do have the local time, they may use that instead of UTC as long as they do so consistently throughout the system. However, this is not recommended since generating the UTC from local time only needs a time-zone offset.

If the clock is set backwards, or if it might have been set backwards (e.g., while the system was powered off), and the UUID generator cannot be sure that no UUIDs were generated with timestamps larger than the value to which the clock was set, then the clock sequence MUST be changed. If the previous value of the clock sequence is known, it MAY be incremented; otherwise it SHOULD be set to a random or high-quality pseudorandom value.

Similarly, if the Node ID changes (e.g., because a network card has been moved between machines), setting the clock sequence to a random number minimizes the probability of a duplicate due to slight differences in the clock settings of the machines. If the value of the clock sequence associated with the changed Node ID were known, then the clock sequence MAY be incremented, but that is unlikely.

The clock sequence MUST be originally (i.e., once in the lifetime of a system) initialized to a random number to minimize the correlation across systems. This provides maximum protection against Node IDs that may move or switch from system to system rapidly.

5.1. UUID Version 1​

5.1. UUID Version 1