IPv6 ULA Generator
Random RFC 4193 unique local /48 prefixes with /64 subnets
Results
ULA addresses are for local and private IPv6 networks. They are not routed on the public Internet.
Options
1–4 hex digits, 0 to ffff.
Advanced
Exactly 10 hex digits, to plan subnets for a prefix you already use.
What it generates
A unique local address (ULA) prefix for private IPv6 networks: fd followed by a random 40-bit Global ID gives your site a /48 such as fd7a:93bf:c120::/48. The generator also shows the Global ID, a /64 subnet for the subnet ID you choose, an example host address, the reverse DNS zone of the /48 and the 65,536 available /64 subnets. Every value has its own copy button.
Why the Global ID is random
RFC 4193 asks for a pseudo-random Global ID so two private networks that are later merged or connected over a VPN are very unlikely to collide. The ID comes from your browser’s cryptographic random generator (crypto.getRandomValues) and is created after the page loads. Regenerate draws a new one; nothing is saved or sent anywhere.
Subnets and existing prefixes
The subnet ID is the fourth hextet: 1 to 4 hex digits from 0 to ffff, so 42 gives fd7a:93bf:c120:42::/64. Under Advanced you can enter the 10-digit Global ID of a ULA prefix you already use to plan its subnets. The legacy MAC address and timestamp method from RFC 4193 is not offered; random IDs meet the same goal without exposing hardware details.
Where ULAs work
Unique local addresses are for communication inside a site or between cooperating sites. They are not routed on the public Internet, so hosts that need Internet access also need a global address or translation. Keep a generated prefix once you start using it: changing it later means renumbering every subnet.
