UUIDv1

UUID version 1 (UUIDv1) is a time-based universally unique identifier defined in RFC 4122. It is designed to be globally unique for identifiers generated across different machines and moments in time. A UUIDv1 is 128 bits and is usually displayed as five hexadecimal groups in the pattern 8-4-4-4-12.

The UUIDv1 layout includes these fields: time_low (32 bits), time_mid (16 bits), time_hi_and_version (16 bits), clock_seq (16 bits), and node (48 bits). The time fields encode a 60-bit timestamp representing the number of 100-nanosecond intervals since the UUID epoch of 00:00:00.00, 15 October 1582. In time_hi_and_version, the most significant four bits are set to the version number 1.

The clock_seq field provides a guard against duplicates when the clock is adjusted or moved backward. The two most significant bits of the clock_seq_hi_and_reserved portion are reserved for the variant and must be set to 10, indicating RFC 4122 variant 1. The remaining 14 bits of the clock_seq act as a counter to preserve uniqueness within the same timestamp.

The node field is a 48-bit value that typically contains the MAC address of the generating node. If a MAC address is not available, a random value can be used, with the multicast bit set to indicate that it is not a real network address.

Because UUIDv1 embeds the generating time and node identity, it supports chronological ordering and can reveal the generating host. This has privacy implications, and for privacy-sensitive applications UUIDv4 or name-based variants may be preferred.