Choose the right IPv6 prefix length for a required number of subnets and see how much headroom is left.
Subnet count is always a power of two, so the required bits are the base-2 logarithm of the target rounded up. Standard practice stops at /64 because SLAAC and many hardware pipelines assume a 64-bit interface identifier. Prefix length is the one IPv6 decision that is expensive to reverse: it is embedded in DNS, ACLs, route filters and monitoring long before anyone revisits it.
IPv6 Prefix Length
subnet bits = ceil(log2(subnets needed × growth)); prefix = site prefix + subnet bits, capped at /64.
subnet bits = ceil(log2(subnets needed × growth)); prefix = site prefix + subnet bits, capped at /64. Subnet count is always a power of two, so the required bits are the base-2 logarithm of the target rounded up. Standard practice stops at /64 because SLAAC and many hardware pipelines assume a 64-bit interface identifier.
Prefix length is the one IPv6 decision that is expensive to reverse: it is embedded in DNS, ACLs, route filters and monitoring long before anyone revisits it.
This calculator takes 3 inputs: Allocated site prefix, Subnets needed today, Planned growth multiple. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.