HomeIP LookupIPv6 ULA

Unique Local IPv6 (RFC 4193)

You landed here because the IPv6 address you looked up sits inside RFC 4193 Unique Local Address space — fc00::/7. ULA is IPv6’s private-use range: it is routable across your own sites but must never appear in the global IPv6 table. No registry delegates it, so there is no WHOIS or geolocation record.

Unique Local Addresses
fc00::/7 (practically fd00::/8)
RFC 4193

Frequently asked questions

What is the difference between ULA and link-local IPv6?

Link-local addresses (fe80::/10) are only valid on a single link — you cannot route them between VLANs, and every link reuses them. ULA (fc00::/7) is routable inside a single organisation across many links and sites, but never on the public internet. ULA gives you the 'private but routable internally' semantics that RFC 1918 gives you on IPv4.

Will fc00::/7 ever be globally routed?

No. RFC 4193 requires transit providers to drop fc00::/7 at their peering boundaries and every production IPv6 filter does. The 'fc' prefix is specifically chosen so it never collides with globally unicast IPv6 (2000::/3) — there is no scenario where ULA traffic can escape your network by accident and be accepted upstream.

How is ULA different from RFC 1918?

ULA is probabilistically unique: RFC 4193 asks you to generate a random 40-bit global ID so that if two organisations merge, their ULA space almost certainly does not collide. RFC 1918 makes no such guarantee — everyone uses 10.0.0.0/8, so corporate mergers routinely trigger painful NAT renumbering projects. ULA avoids that because every organisation picks a different /48 prefix.

What is the difference between fc00::/8 and fd00::/8?

RFC 4193 splits fc00::/7 in half. fd00::/8 is for locally-assigned ULA — you pick a random /48 out of it and nobody coordinates with anyone. fc00::/8 is reserved for centrally-assigned ULA that never got standardised, so in practice all ULA traffic you will ever see lives in fd00::/8.

Should I use ULA for my home network?

If your ISP gives you a stable delegated prefix (DHCPv6-PD), prefer that — you get real global addressing without NAT. ULA makes sense if your ISP rotates your prefix, if you want stable internal addressing that survives an ISP swap, or if you want to run IPv6 on an air-gapped network. Many home users run ULA in parallel with their ISP's global prefix so internal services stay reachable across prefix changes.

Related reading