Free Subnet Finder

Last reviewed: May 2026

Every other subnet calculator tells you what a subnet is. This one tells you where the next one goes. Paste a parent block and the subnets already allocated inside it, and get every free block, where a new subnet of a given size fits, and how much space is left.

How the finder works

Finding free space by hand is error-prone because allocations are rarely in order and often overlap. The algorithm here is the same one an IPAM system uses:

1. Convert the parent block and every allocation to a numeric start–end range.
2. Discard allocations that fall outside the parent, and clip any that straddle its edge.
3. Sort by starting address, then merge ranges that overlap or sit directly next to each other.
4. Read off the gaps between the merged ranges — that is the free space.
5. Break each gap into the largest CIDR-aligned blocks that fit, since a gap is not always expressible as a single subnet.

Step 5 matters: the range 10.0.4.0 – 10.0.7.255 is exactly one /22, but 10.0.4.0 – 10.0.9.255 is not a single subnet at all — it is a /22 plus a /23. The finder always returns valid, correctly aligned CIDR blocks you can paste straight into Terraform or your IPAM.

Why fragmentation matters more than utilisation

A block can be only 40 % utilised and still have nowhere to put a /20. If allocations are scattered, the free space is chopped into small pieces and the largest usable block may be far smaller than the total free count suggests. That is why this tool reports largest free block next to total free addresses — the two together tell you whether you need to re-plan before you run out.