Build an RFC 4193 unique local address prefix from a global ID and see how many subnets it yields.
A ULA prefix is fd followed by 40 randomly chosen bits, which produces a /48 that is almost certainly unique without registration. The next 16 bits give 65,536 /64 subnets for internal use. ULAs give stable internal addressing that survives an ISP change, but only if the global ID is genuinely random — sequential IDs collide the first time two networks merge.
IPv6 ULA
prefix = fd (fc00::/7 with the local bit set) + a 40-bit pseudo-random global ID, leaving 16 bits of subnet ID inside a /48.
prefix = fd (fc00::/7 with the local bit set) + a 40-bit pseudo-random global ID, leaving 16 bits of subnet ID inside a /48. A ULA prefix is fd followed by 40 randomly chosen bits, which produces a /48 that is almost certainly unique without registration. The next 16 bits give 65,536 /64 subnets for internal use.
ULAs give stable internal addressing that survives an ISP change, but only if the global ID is genuinely random — sequential IDs collide the first time two networks merge.
This calculator takes 3 inputs: Global ID (10 hex digits), Subnet ID (hex), Subnets needed. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.