Subnet calculator
Lists the subnets of an IPv4 or IPv6 block with the new prefix length, in order. The new prefix must not be shorter than the block's and at most 256 subnets are returned; a larger split is refused before anything is computed.
Updated 2026-10-27
What this tool checks
The tool splits the block into subnets of the new prefix length and lists them from the lowest address up. A /24 split into /26 gives four subnets of 64 addresses each.
How to read the result
Each line is one subnet in CIDR form. To see the addresses one subnet covers, use the CIDR calculator; to turn an arbitrary range into blocks, use IP range to CIDR.
Limitations
- At most 256 subnets per request.
- Nothing is looked up: the tool does not know which subnets are in use.
Use it from code
API reference for Subnet calculator
curl -i "https://api.lookproof.dev/v1/ip/subnets?cidr=192.0.2.0%2F24&prefix=26"A request without payment is answered with 402 Payment Required and the exact price. See the API reference.
Limits
| Input | Network (CIDR), New prefix length |
|---|---|
| Deadline | 2 seconds |
| Freshness | Computed from the input; nothing is looked up |
| Price | $0.002 per check |
Findings this tool can report
This tool reports no findings: the result is the computed data.
Frequently asked questions
Why at most 256 subnets?
Splitting a large block can give an enormous number of subnets: an IPv6 /0 into /128 would be 2 to the power of 128. The limit keeps every answer small, and a split that would exceed it is refused before anything is computed.
Which prefix lengths can I use?
The new prefix must be at least the block's own prefix and at most 32 for IPv4 or 128 for IPv6, and at most 8 longer than the block's prefix.
What if the block has host bits set?
The block is read as its network, so 192.0.2.5/24 is split like 192.0.2.0/24.