IPv6 Prefix Length Calculator
Choose the right IPv6 prefix length for a required number of subnets and see how much headroom is left.
Inputs
Recommended Subnet Prefix
59
Subnets Available
2,048
Headroom vs Requirement
582.7%
Subnet Bits Used
11
Step by step
Values used
Allocated site prefix = 48 bits; Subnets needed today = 300 subnets; Planned growth multiple = 4 x
IPv6 Prefix Length
subnet bits = ceil(log2(subnets needed × growth)); prefix = site prefix + subnet bits, capped at /64.
Recommended Subnet Prefix
= 59
Subnets Available
= 2,048
Headroom vs Requirement
= 582.7
Subnet Bits Used
= 11
How it works
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.
Formula
IPv6 Prefix Length
subnet bits = ceil(log2(subnets needed × growth)); prefix = site prefix + subnet bits, capped at /64.
- site
- Prefix length allocated to the site
- bitsUsed
- Bits spent on the subnet field
Frequently Asked Questions
How is IPv6 Prefix Length calculated?
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.
Why does IPv6 Prefix Length matter?
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.
What values do I need to enter?
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.