Split CIDR into N Equal Subnets

Splitting an address range into equal parts is the calculation that shows up every time someone builds a VPC, separates production from staging or hands blocks out to branch offices. The rule is short: every time the prefix grows by one bit, the block halves. A /24 becomes two /25s, four /26s, eight /27s, and onwards.

Enter the base block and how many subnets you want and the page returns the list, with the first and last address of each. The number of parts has to be a power of two, because the division happens by bits — there is no splitting a block into three equal parts in CIDR, and the page refuses rather than rounding silently. It also refuses when the resulting prefix would exceed 32.

Two sums regularly cause confusion. A block's address count is two to the power of thirty-two minus the prefix, but not all of them are usable: in an ordinary subnet the first is the network address and the last is broadcast, which leaves a /24 with 254 hosts rather than 256. And cloud providers reserve a few more in every subnet — AWS, for instance, takes five.