UUID v5 Generator

About UUID Generator

All seven versions are the same 128 bits arranged differently, and the arrangement is the whole decision. v4 is 122 random bits and encodes nothing. v7 puts a millisecond timestamp in front of the randomness so identifiers sort chronologically, which is what a database index wants. v5 and v3 are not generated at all — they are derived by hashing a namespace and a name, so the same input always gives the same UUID on any machine, for ever.

v1 and v2 come from an older idea of what an identifier should be. v1 carries a timestamp accurate to a tenth of a microsecond and a node field meant to hold the machine's MAC address — a combination that has been used to trace documents back to the computer that produced them, and the reason the version fell out of favour. v2 goes further and embeds a local user or group id, spending the low word of the timestamp to do it. Both are here so you can read them; for new work v7 gives the same ordering with none of the baggage.

Reading one back is the other half of this tool, and usually the more interesting half. The version digit is the part everybody looks at and the least of what is in there — a v1 will tell you when it was made, to what precision, and whether its node field is a real network card or a random stand-in.

Where it helps

• Every version in common use, plus the two reserved values, generated or read.
• Reading one reports what it actually encodes — the timestamp, the node, the clock sequence — not just the version digit.
• Cryptographically secure randomness throughout, generated locally in this browser.
• Up to 500 at once for seeding fixtures or test data, in three output formats.

Limits

• v1, v2, v6 and v7 all disclose their creation time, and v1 and v2 may disclose the machine as well.
• 16 bytes per id is heavier than a bigint primary key, in both storage and index size.
• UUIDs are unguessable but not secret; do not treat one as an access token.
• v2 is barely implemented anywhere — most libraries decline it, and RFC 9562 lists it as reserved rather than recommending it.

Frequently asked questions

v4 or v7?

v7 for database primary keys and anything sorted by creation. v4 when the id must reveal nothing at all, including when it was made.

Should I ever use v1 now?

Only to interoperate with something that already does. v7 gives the same time ordering without the node field, and v6 exists if you need v1's layout with sortable ordering.

Can I get the name back out of a v5?

No — it is a hash, so the name is not recoverable. But it is confirmable: anyone who guesses the name can check the guess with a single hash.

What does the multicast bit tell me?

In a v1 or v2 node field, set means the node was randomly generated and discloses no hardware; clear means it is very likely a real MAC address. It is the first thing to check when auditing what an identifier leaks.

Could two UUIDs ever collide?

Not for v4 or v7 in any practical sense — a collision needs around 2.7 quintillion of them. The versions worth worrying about are the derived ones, where the same name always gives the same value by design.

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