Generate UUIDs

No. Every UUID is generated entirely on your device using your browser’s own cryptographically secure random number generator, and nothing is sent anywhere. Turn off your Wi-Fi after the page loads and it still works. This page loads 3KB of JavaScript, gzipped - about what your browser's network tab will show for this page's own scripts.

For most new IDs, v4 (fully random) or v7 (time-ordered, so IDs created later sort after IDs created earlier - useful as a database primary key) are the common defaults. Use v1 if you specifically need the older time-based format some existing systems expect. Use v5 if you need the exact same input (a namespace plus a name, like a URL or domain) to always produce the exact same UUID - useful for deduplicating or cross-referencing records by a stable identifier instead of generating a new random one every time.

RFC 4122’s v1 format was designed around a device’s real network (MAC) address and a clock sequence persisted across calls, to keep IDs unique even if the clock moves backward. This tool has neither a MAC address to read nor anywhere to persist state between page loads, so - as the RFC itself allows as an alternative - it uses a fresh, randomly generated node ID (with the bit set that marks it as not a real MAC address) and a fresh random clock sequence on every UUID. Combined with the timestamp, that still makes a collision astronomically unlikely, but it is not a drop-in replacement for a long-running server’s stateful v1 generator.

RFC 4122 defines four standard namespaces for common input types - DNS (for domain names), URL, OID, and X.500 - all offered here as presets. If you’re hashing something else entirely (an internal system’s own identifier scheme, for example), enter that system’s own namespace UUID as a custom namespace instead; the important thing is that everyone hashing the same kind of name uses the same namespace, since the namespace is part of what gets hashed.

Practically, yes. A v4 UUID has 122 random bits - even generating billions of them, the odds of two ever matching are negligible. v1 and v7 add a real timestamp on top of similar randomness. v5 is the one deliberate exception: it’s a hash, so the exact same namespace and name always produce the exact same UUID on purpose, which is the point of using it.