IPv6 ULA Generator (RFC 4193) — Free
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Unique local addresses are the IPv6 equivalent of private ranges: routable inside your network, useless outside it. Enter a MAC address and this tool applies the first RFC 4193 method — timestamp plus MAC, SHA-1 hashed, low 40 bits — to mint your fd00::/48 block along with the first and last routable /64 subnets.
How it works
- 1
Enter a MAC address
Any device MAC seeds the generator.
- 2
Generate the block
Timestamp plus MAC are hashed per RFC 4193.
- 3
Copy your /48
Take the ULA block and its first and last routable subnets.
About this tool
What a ULA is
Unique Local Addresses are IPv6's equivalent of private IPv4 ranges: fc00::/7 addresses that route inside your organization but not on the public internet. The generator produces fd00::/8 prefixes — the currently sanctioned half — with a random 40-bit global ID.
Why the random global ID
ULA prefixes are meant to be unique even between unconnected organizations — the RFC specifies a random, pseudo-randomly generated 40 bits so two networks that merge later do not collide. This is the one part of the address you should never hand-pick sequentially.
ULA versus link-local versus public
fe80:: link-local addresses exist per-interface and never route; ULAs route internally but not on the internet; public (global unicast) addresses route everywhere. ULAs fill the middle: stable internal addressing that survives router changes without exposing anything publicly.
Frequently asked questions
Should I use ULAs instead of IPv4 private ranges?
For IPv6-internal networks, they serve the same role with 64-bit subnets and no NAT — the standard guidance is ULA internally, public globally, no translation games.
Why must the global ID be random?
To guarantee uniqueness between organizations without a registry. Sequential prefixes work until two networks merge or connect — then the collision is a renumbering project.
Can a ULA reach the internet?
Not directly — internet routers drop fc00::/7. That is by design; reach the internet from a separate global-unicast address.