What is a UUID?
A UUID (universally unique identifier) is a 128-bit label written as 32 hexadecimal characters in five groups, like 123e4567-e89b-42d3-a456-426614174000. Software uses UUIDs to name things without checking a central list, because two matching is extremely unlikely.
Action level No action usually needed
- Why you are seeing it
- It appears as a 32-character code in five groups, in software and data that need a unique name for each item.
- Why it matters
- Software uses it to name things without checking a central list, because two matching is extremely unlikely.
- What should I do?
- Read the groups as 8, 4, 4, 4 and 12 characters; the first digit of the third group is the version.
- Do not use a random UUID as a password or a security token.
- A common misunderstanding
- A random UUID is not a secret, so it is not a safe replacement for a password or a token.
Read a UUID, or make a new one
Paste any UUID to see its parts, or generate a random one. Nothing is sent anywhere.
123e4567-e89b-42d3-a456-426614174000
- Written out
- 123e4567-e89b-42d3-a456-426614174000
- Version
- 4: random
- Layout (variant)
- RFC 9562 (the standard layout)
- Size
- 128 bits: 32 hex characters plus 4 dashes
- The first digit of the third group is the version; the first digit of the fourth group tells which layout is used. The rest is random in a version 4 UUID.
- A random UUID is not guaranteed to be unused: the chance that two collide is simply extremely small. A UUID is an identifier, not a secret or a password.
Why developers use them
- A UUID can be made on any computer, even offline, without asking a central counter for the next number.
- Two systems can create records at the same time and merge them later without their labels clashing.
- A random UUID is not a secret. Do not use one as a password or a security token.
What the digits mean
A UUID is written in groups of 8, 4, 4, 4 and 12 characters. The first digit of the third group is the version. The first digit of the fourth group shows which layout the UUID follows; the standard layout starts with 8, 9, a or b.
A simple example
Reading 123e4567-e89b-42d3-a456-426614174000
The third group starts with 4, so it is version 4 (random).
The fourth group starts with a, which is the standard layout.
A version 4 UUID has 122 random bits (128 minus 6 fixed bits), which is why a repeat is so unlikely.
Common versions
| Version | Made from |
|---|---|
| 1 | A timestamp plus a machine value |
| 3 | A name, hashed with MD5 |
| 4 | Random numbers (the most common) |
| 5 | A name, hashed with SHA-1 |
| 6 | A reordered timestamp |
| 7 | A Unix-time timestamp plus random numbers, so new values sort in order |
| 8 | Custom, defined by whoever makes it |