IPv6 Prefix Capacity Calculator
Count available child subnets.
IPv6 Prefix Capacity Calculator: inputs and results
Enter an IP address, CIDR prefix, or range. Then check its first and last address below.
Calculated Boundaries & ParametersReady
Key Capabilities
- •100% Client-Side: Zero telemetry or server latency.
- •RFC Compliant: Exact 32-bit and 128-bit integer precision.
- •Quick Copy: Instant access to CIDR, mask, and range bounds.
- •Cross-Platform: Works on desktop, tablet, and mobile browsers.
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How IPv6 Prefix Capacity Calculator Works?
This tool works by raising 2 to the difference between the child prefix length and the parent prefix length, which counts the child subnets inside the parent.
Enter the parent prefix
Type a length such as
/48.Enter the child prefix
Type the subnet length, such as
/64.Compute the difference
64 − 48 = 16 bits.
Return the count
2^16 = 65,536 child subnets.
IPv6 Prefix Capacity Calculator Formula
The formula is Child Subnets = 2^(c − p), where p is the parent length and c is the child length.
Child count
S = 2^(c − p)Requires c ≥ p.
Hex digits per step
4 bits = 1 hex digit = 16× capacityNibble-aligned plans grow in steps of 16.
Addresses per child
2^(128 − c)2^64 for a /64.
Worked example: /32 into /48 and /56
/32 → /48 = 2^16 = 65,536 site prefixes /32 → /56 = 2^24 = 16,777,216 customer prefixes /32 → /64 = 2^32 = 4,294,967,296 LAN subnets
IPv6 Prefix Capacity Calculator Conversion Table
The table shows how many /48, /56, /60, and /64 subnets fit in 7 parent prefix sizes.
| Parent | /48 Subnets | /56 Subnets | /60 Subnets | /64 Subnets |
|---|---|---|---|---|
| /29 | 524,288 | 134,217,728 | 2,147,483,648 | 34,359,738,368 |
| /32 | 65,536 | 16,777,216 | 268,435,456 | 4,294,967,296 |
| /36 | 4,096 | 1,048,576 | 16,777,216 | 268,435,456 |
| /40 | 256 | 65,536 | 1,048,576 | 16,777,216 |
| /44 | 16 | 4,096 | 65,536 | 1,048,576 |
| /48 | 1 | 256 | 4,096 | 65,536 |
| /56 | — | 1 | 16 | 256 |
What is IPv6 Prefix Capacity Calculator?
The IPv6 Prefix Capacity Calculator is a tool that counts how many child subnets of a chosen size fit inside a parent IPv6 prefix.
IPv6 planning measures capacity in subnets, not in hosts. A /48 holds 65,536 /64 subnets, and a /56 holds 256. Nibble boundaries (/48, /52, /56, /60, /64) multiply capacity by 16 per step and keep prefixes readable in hex.
RIR policies, such as RIPE NCC and ARIN rules, size allocations by the number of /48 or /56 customer prefixes an operator serves. The calculator converts those counts into prefix sizes and back.
What are the uses of IPv6 Prefix Capacity Calculator?
There are 4 main uses of the IPv6 Prefix Capacity Calculator, including RIR requests, customer pool sizing, and site planning.
Justifying RIR requests
An ISP with 200,000 customers at /56 needs more than a /40 (65,536 × /56) and fits a /32 (16.7 million × /56).
Sizing customer pools
Broadband pools are sized by the number of /56 or /60 delegations.
Planning enterprise sites
A /44 gives 16 site prefixes of /48 each.
Comparing nibble options
Planners compare /52 and /56 site sizes before committing to a scheme.