IPv6 Prefix Length Calculator
Choose the right IPv6 prefix length for a required number of subnets and see how much headroom is left.
About IPv6 Prefix Length Calculator
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
Worked example
With the default values below, IPv6 Prefix Length Calculator returns Recommended Subnet Prefix of 59. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Allocated site prefixaccepts 16–96 bits
- 48 bits
- Subnets needed todayaccepts 1–1,000,000 subnets
- 300 subnets
- Planned growth multipleaccepts 1–64 x
- 4 x
Results
- Recommended Subnet Prefix
- 59
- Subnets Available
- 2,048
- Headroom vs Requirement
- 582.7%
- Subnet Bits Used
- 11
Step by step
- 1Values used
- 2IPv6 Prefix Length
- 3Recommended Subnet Prefix= 59
- 4Subnets Available= 2,048
- 5Headroom vs Requirement= 582.7
- 6Subnet Bits Used= 11
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.