IPv6 ULA Calculator
Build an RFC 4193 unique local address prefix from a global ID and see how many subnets it yields.
Inputs
ULA Prefix
fda1:b2c3:d4e5::/48
Example /64 Subnet
fda1:b2c3:d4e5:0001::/64
/64 Subnets Available
65,536
Subnet Space Used
0.76%
Step by step
Values used
Global ID (10 hex digits) = a1b2c3d4e5; Subnet ID (hex) = 0001; Subnets needed = 500 subnets
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.
ULA Prefix
= fda1:b2c3:d4e5::/48
Example /64 Subnet
= fda1:b2c3:d4e5:0001::/64
/64 Subnets Available
= 65,536
Subnet Space Used
= 0.76
How it works
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.
Formula
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.
- fd
- fc00::/7 with the L bit set to 1, the only ULA form in use
- global ID
- 40 bits that must be randomly chosen, not sequential
Frequently Asked Questions
How is IPv6 ULA calculated?
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.
Why does IPv6 ULA matter?
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.
What values do I need to enter?
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.