CIDR calculator

CIDR replaced the old class system in 1993 and the reason was arithmetic. Classes fixed the network boundary at 8, 16 or 24 bits, so an organisation needing 300 addresses had to take a class B — 65,534 of them — and waste the rest. CIDR lets the boundary sit anywhere, so 300 hosts get a /23 and 512 addresses.

The rule to internalise is that a longer prefix is a smaller network, which reads backwards the first hundred times. A /24 is 256 addresses; a /25 is 128. Each bit added to the prefix halves the block. That is also why the counts are all powers of two and why you cannot have a network of 300 addresses — you take 512 and use 300.

The classes are still shown here because people still say "a class C" when they mean a /24, and because the class of an address determines its default mask on equipment old enough to care. They have had no effect on routing for thirty years.

One thing worth watching in real use: a prefix must be aligned to its own size. 10.0.1.0/23 is not a valid network, because a /23 must start on an even third octet — the correct block is 10.0.0.0/23. Equipment will usually reject it; a calculator that accepts it and computes the containing network, as this one does, tells you what you actually asked for.