CIDR/Subnet Splitter

How it works

Splitting a block means borrowing extra bits from the host portion into the network prefix. Going from a base prefix of /P to a target prefix of /Q borrows Q−P bits, and each borrowed bit doubles the number of pieces, so the block divides into exactly 2^(Q−P) equal-sized subnets, each with 2^(32−Q) addresses. The tool walks through each of those subnets in order, adding the block size to the previous network address (via 32-bit integer math, same as the subnet calculator's ip-to-int/int-to-ip helpers) to get the next one, then computing each subnet's broadcast, first/last usable host, and usable host count exactly the way a single-subnet lookup would.

FAQ

Why is the usable host count 2 less than the subnet size?

Every subnet reserves its first address as the network address and its last as the broadcast address, so a /26 with 64 total addresses has 62 usable hosts — except at /31 and /32, where the block is 2 addresses or fewer and usable hosts is shown as 0 rather than going negative, since there's no separate network/broadcast pair to reserve.

What happens if the number of subnets I need isn't a power of two?

CIDR splitting only produces equal-sized blocks in powers of two, so when you ask for a subnet count, the tool rounds up to the smallest prefix length that yields at least that many subnets — meaning some of the resulting subnets may be extra and unused.