What is a UUID?

A UUID (Universally Unique Identifier), also known as a GUID (Globally Unique Identifier), is a 128-bit identifier standardised in RFC 4122 and displayed as five groups of hexadecimal digits separated by hyphens: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx

The M nibble encodes the version and the two most-significant bits of N encode the variant.

UUID versions

Version	Method	Sortable?	Contains timestamp?
v1	Time-based + MAC address	No	Yes (Gregorian)
v2	DCE Security	No	Yes
v3	MD5 name hash	N/A	No
v4	Random	No	No
v5	SHA-1 name hash	N/A	No
v6	Reordered time (draft)	Yes	Yes (Gregorian)
v7	Unix-time + random	Yes	Yes (Unix ms)

Special UUIDs

Nil UUID: All zeros (00000000-0000-0000-0000-000000000000). Used as a sentinel or default value.
Max UUID: All ones (ffffffff-ffff-ffff-ffff-ffffffffffff). Defined in RFC 9562 as the highest possible UUID value.

v1 timestamp extraction

Version 1 UUIDs embed a 60-bit timestamp counting 100-nanosecond intervals since 15 October 1582 (the Gregorian calendar epoch). This tool decodes that timestamp and converts it to a human-readable UTC date.

The node field in v1 UUIDs was originally the MAC address of the generating machine. For privacy reasons, modern implementations often substitute a random node value.

v1 MAC address privacy risk

The original UUID v1 specification used the MAC address of the network interface that generated the UUID as the node field (the last 12 hex characters). This exposed the hardware identity of the generating machine, allowing UUIDs to be traced back to a specific computer. This was a significant privacy concern - and one key reason why UUID v4 (fully random) became the dominant version. Modern v1 implementations typically substitute a random node to mitigate this risk.

v7 timestamp extraction

UUID v7 stores a Unix millisecond timestamp in the most significant 48 bits. For a v7 UUID like 018f4a2c-3b00-7abc-8def-123456789abc:

The first three hyphen-separated groups encode the timestamp: 018f4a2c-3b00 is a 48-bit value.
Convert to decimal: 0x018f4a2c3b00 = 1,716,812,000,000 ms -> a date in May 2024.

This tool extracts and displays the embedded v7 timestamp automatically.

Namespace UUIDs for v3 and v5

UUID v3 (MD5) and v5 (SHA-1) are name-based: they hash a namespace UUID and a name string together to produce a deterministic UUID. The same namespace + name always produces the same UUID. Pre-defined namespace UUIDs from RFC 4122:

DNS: 6ba7b810-9dad-11d1-80b4-00c04fd430c8
URL: 6ba7b811-9dad-11d1-80b4-00c04fd430c8
OID: 6ba7b812-9dad-11d1-80b4-00c04fd430c8
X.500 DN: 6ba7b814-9dad-11d1-80b4-00c04fd430c8

Example: UUID v5 with the URL namespace and name https://example.com always produces 2ed6657d-e927-568b-95e3-af7ca03ece37. This makes v3/v5 ideal for generating stable, reproducible IDs from known inputs such as content URLs or canonical record keys.