Split a block and find the free space around it.

Give it a parent block — for example 10.0.0.0/24 — plus any subnets already allocated inside it, and it subtracts the used space and reports the gaps as a minimal set of CIDRs. It can also split the parent (or any free gap) evenly into /N subnets and tell you the next free block of a size you ask for. Everything runs in your browser; nothing is uploaded.

No. Parsing and all the maths run 100% client-side in your tab — there is no server, no account and no logging. You can safely paste internal parent blocks and allocations.

Free space is the part of the parent block that none of your existing allocations cover. Because that leftover region rarely lines up with a single power-of-two boundary, the splitter expresses it as a Variable-Length Subnet Mask (VLSM) set — the smallest list of differently-sized CIDRs that exactly tiles the gap, rather than forcing one fixed prefix.

It scans the free space in order and returns the first block of prefix /N that is fully unallocated and correctly aligned to its own boundary — the block you would hand out next if you were carving the parent into /N subnets. If no aligned /N fits in the remaining free space, it tells you so instead of guessing.

Yes. IPv6 is handled with full 128-bit precision using BigInt, so you can split a site /48 into /64s or find the free space inside a /56 just as accurately as you subtract /26s from an IPv4 /24. IPv4 and IPv6 parents are both first-class.

Enter the parent CIDR — for example 10.0.0.0/16 — and choose a new prefix length such as /24. The tool enumerates every equal-size subnet of that prefix within the parent, marking each one as free, used, or partially allocated. Splitting a /16 into /24s yields 2^(24-16) = 256 subnets, each with its own network address and 254 usable host addresses.

Provide the parent CIDR, enter your existing allocations, and select the target prefix length. The tool scans the free intervals left after subtracting used space and returns the first correctly aligned block of that size that does not overlap any existing allocation. This is the next available subnet you can hand out without causing a conflict.

Paste your existing allocations under a parent CIDR and the tool subtracts the used intervals from the parent range. It then expresses each unallocated region as the minimal set of aligned CIDRs, so you can see every gap — including the largest contiguous free block — at a glance.

Use 2^(new prefix minus parent prefix): 2^(24-16) = 256. A /16 contains 256 /24 subnets, each with 256 total addresses (254 usable after the network and broadcast addresses). You can verify this by entering 10.0.0.0/16 with a split prefix of /24 and seeing all 256 blocks listed.

Overlapping subnets cause routing conflicts because one address would appear to belong to two ranges. The tool checks each candidate subnet against your existing allocations and marks it as used or partial if it conflicts, only flagging a subnet as free when it is fully unallocated and correctly aligned within the parent CIDR.