UUID Generator

What UUID Generator does

It makes UUIDs, in every version a browser can honestly produce, and reads one back to tell you what it is. Pick a version, pick how it should be written, say how many you want, and they are generated here — from your browser’s cryptographic randomness, never from Math.random.

A GUID is not a different thing from a UUID; it is the same 128 bits with Microsoft’s punctuation. So the braces and the capital letters are a choice about how the value is written, alongside canonical form, no hyphens at all, and a URN.

When you need a name nothing else has

When a row needs a primary key before the database has seen it. When two systems have to agree on an identifier without asking each other. When a file, a request or an upload needs a name that will not collide with the last one. Version 4 covers nearly all of it, and version 7 is the one to reach for when the identifier is going into an index.

The other half is reading. A UUID in a log, a filename or a database is a small artefact with facts inside it: which version it is, when it was made if it carries a time, and — for a version 1 — whether the machine that made it signed its own network address into it. Paste one in and it will say.

Which version, and what each one tells the world

The versions are not variations on a theme. They differ in what goes into the 128 bits, which means they differ in what someone holding one can work out.

Version 4 — random

One hundred and twenty-two bits of randomness and six bits saying which version it is. It reveals nothing, because there is nothing in it but randomness. Unless you have a reason to want something else, this is the one.

Version 7 — time-ordered

Milliseconds since 1970 in the first six bytes, then a counter, then randomness. Two of them sort into the order they were made, which is why it has become the recommended choice for a database key: the index grows at one end instead of taking a random write every insert. The cost is that the creation time is in plain sight, by design.

Why a batch of version 7s still sorts

A millisecond is a long time, and forty of these generated at once would share every ordered bit and differ only in randomness — so a batch would come out shuffled, which is the case that actually happens when something inserts a lot of rows. The twelve bits after the timestamp are therefore the monotonic counter RFC 9562 provides for it, advancing within a millisecond and reseeded when the millisecond turns. Ask for a thousand here and they are in order, not merely near it.

Versions 1 and 6 — a clock and a node

The 1997 design: a timestamp counting hundred-nanosecond intervals since 1582, a clock sequence, and a node. Version 6 is the same fields rearranged so the value sorts by time. Generate these when something old expects them.

Why a version 1 from here cannot identify your machine

A version 1 was meant to carry the network card’s MAC address, which is why one found in a document can point at the computer that made it. A browser cannot read a MAC address at all, so the node here is random with its multicast bit set — the flag that means "not a real address". Read one of these back and it will tell you so. It is also how you can tell, of a version 1 you were given, whether somebody’s machine is written into it.

Versions 3 and 5 — derived from a name

Not random at all: the digest of a namespace and a name, so the same name always produces the same UUID, on any implementation. Version 5 uses SHA-1 and is the one to use; version 3 uses MD5 and exists so you can reproduce identifiers a system already made that way. Ask for ten and you get ten copies of one value, because that is what these versions are.

Nil and Max

All zeroes and all ones. The Nil is a defined way of saying "no UUID"; the Max was added by RFC 9562 as its opposite. Both are mostly useful for finding out whether something checks its input.

How many at once

Up to 1,000 in one run, copied together or downloaded one per line. It is text, so the limit is about the size of a sensible page rather than about memory.

Nothing is uploaded, and nothing about your machine goes in

The identifiers are made in this browser and nothing is sent anywhere — there is nowhere to send it, because this page has no endpoint behind it. The name you type for a version 3 or 5 is never uploaded either.

It is worth being precise about the second half of that, because one UUID version was designed to embed hardware identity. There is no MAC address in anything this page produces: a browser cannot read one, so the node field is random and flagged as random. Nothing here is stored between visits, including the clock sequence, which is generated fresh each time rather than remembered.