Break an IPv6 prefix into its network portion, available subnet count and address capacity.
The network portion is produced by masking every 16-bit group down to the prefix length. Subnet count is the difference in prefix lengths as a power of two, and each subnet keeps 128 minus its own prefix bits for interfaces. IPv6 allocations are handed out as /48 or /56 blocks and the whole design depends on knowing how many /64s that yields before addresses are burned into DNS and firewall policy.
IPv6 Prefix
subnets = 2^(target − prefix); addresses per subnet = 2^(128 − target).
subnets = 2^(target − prefix); addresses per subnet = 2^(128 − target). The network portion is produced by masking every 16-bit group down to the prefix length. Subnet count is the difference in prefix lengths as a power of two, and each subnet keeps 128 minus its own prefix bits for interfaces.
IPv6 allocations are handed out as /48 or /56 blocks and the whole design depends on knowing how many /64s that yields before addresses are burned into DNS and firewall policy.
This calculator takes 3 inputs: IPv6 address or prefix, Prefix length, Subnet size to carve. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.