Local IPv6 Address Generator
Generate a private, collision-resistant Unique Local Address (ULA) prefix for your internal IPv6 network - no registration required.
Unique Local IPv6 prefix (RFC 4193)
Generates a private fd00::/8 prefix with a cryptographically random 40-bit global ID (crypto.getRandomValues). Random selection keeps the odds of a collision with another site's ULA prefix vanishingly small - useful when networks later merge.
About Local IPv6 Address Generator
A Unique Local Address is the IPv6 equivalent of the familiar private IPv4 ranges such as 10.0.0.0/8 and 192.168.0.0/16. Defined in RFC 4193, ULAs live in fc00::/7; the half that is available for you to assign yourself is fd00::/8. These addresses are meant for internal use and are filtered by providers, so they are never routed on the public internet - they give you stable, self-owned IPv6 addressing that does not depend on any registry or ISP.
The whole point of a ULA is that its global ID should be picked at random. This generator builds a prefix from fd plus a 40-bit global ID drawn from a cryptographically secure random source (the browser's crypto.getRandomValues). Random selection - rather than sequential or 'nice-looking' IDs - is what makes the odds of clashing with another organisation's ULA prefix vanishingly small, which is exactly what lets two independently addressed networks merge later without having to renumber.
The output is laid out so you can put it straight to work: a /48 site prefix that represents your whole organisation, an example /64 subnet, an example host address, and the full address range the prefix covers. A /48 gives you a 16-bit subnet ID, which is 65,536 separate /64 subnets - and each /64 still holds a complete 64-bit interface-ID space, so you never run short of host addresses.
Generation happens entirely in your browser. The random prefix is created locally and never transmitted, so you can produce internal addressing plans privately, then click again any time you want a fresh random global ID.
How to use it
- 1Click Generate to create a random RFC 4193 ULA prefix using your browser's secure random source.
- 2Copy the /48 site prefix - this is your organisation's private IPv6 block.
- 3Carve /64 subnets from it using the 16-bit subnet ID; the example /64 shows the pattern to follow.
- 4Assign host addresses within any /64 as needed, and keep the generated prefix documented for reuse.
- 5Regenerate whenever you want a different random global ID.
Common use cases
- -Giving an internal network stable IPv6 addressing that survives changes to an ISP-assigned prefix.
- -Addressing home labs, test environments, and datacenter back-end networks that must not be reachable from the internet.
- -Choosing space that stays valid when two sites or companies merge, since random global IDs almost never collide.
- -Numbering VPNs and site-to-site links with private, non-conflicting IPv6 addresses.
- -Building a documented IPv6 addressing plan without applying to a registry.
Frequently asked questions
- What is a unique local IPv6 address (ULA)?
- A ULA is a privately scoped IPv6 address in fc00::/7 - in practice fd00::/8 - defined by RFC 4193. It is the IPv6 counterpart of private IPv4 ranges: routable within an organisation but never on the public internet.
- Why should a ULA global ID be random?
- RFC 4193 requires a pseudo-random 40-bit global ID so that prefixes generated independently almost never collide, which lets separate networks merge without renumbering. This tool draws that ID from a cryptographically secure random source.
- How many subnets does a /48 ULA give me?
- A /48 prefix provides a 16-bit subnet ID, which is 65,536 separate /64 subnets. Each /64 still contains a full 64-bit interface-ID space for host addresses.
- Can ULA addresses reach the internet?
- No. Providers filter fc00::/7, so ULAs are strictly for internal use. To reach the public internet you still need a globally routable (GUA) prefix assigned by your ISP or registry.
- Should I just make up an fd00:: prefix myself?
- No. Choosing a non-random or memorable prefix defeats the collision-avoidance design and risks clashing with another network. Always generate a random 40-bit global ID, as this tool does automatically.
- Is the generated prefix sent to a server?
- No. The prefix is created in your browser using local randomness and is never transmitted, logged, or stored anywhere.