IPv6 Prefix Calculator
Break an IPv6 prefix into its network portion, available subnet count and address capacity.
Inputs
Network Prefix
2001:db8:acad::/48
Subnets Available
65,536
Host Bits per Subnet
64
Addresses per Subnet
18,446,744,073,709,552,000
Step by step
Values used
IPv6 address or prefix = 2001:db8:acad::; Prefix length = 48 bits; Subnet size to carve = 64 bits
IPv6 Prefix
subnets = 2^(target − prefix); addresses per subnet = 2^(128 − target).
Network Prefix
= 2001:db8:acad::/48
Subnets Available
= 65,536
Host Bits per Subnet
= 64
Addresses per Subnet
= 18,446,744,073,709,552,000
How it works
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.
Formula
IPv6 Prefix
subnets = 2^(target − prefix); addresses per subnet = 2^(128 − target).
- prefix
- Length of the allocated prefix
- target
- Prefix length of each carved subnet
Frequently Asked Questions
How is IPv6 Prefix calculated?
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.
Why does IPv6 Prefix matter?
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.
What values do I need to enter?
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.