Subnet calculator

A subnet mask is a run of ones followed by a run of zeros, and everything else follows from that. The ones mark the network part of the address and the zeros the host part, so ANDing an address with its mask gives the network address, and ORing it with the inverted mask gives the broadcast. That is the whole calculation — the rest is presentation.

The usable count is total minus two, because the all-zeros host is the network itself and the all-ones host is the broadcast. Two prefixes break that rule and both are common in real configurations. A /31 has no broadcast at all: RFC 3021 defines it as a point-to-point link with two usable addresses, which is why router-to-router links are configured that way and why a calculator reporting "0 usable hosts" for a /31 is wrong. A /32 is a single host — a host route rather than a network.

One implementation note that explains a lot of broken calculators. JavaScript’s bitwise operators return a signed 32-bit integer, so any address from 128.0.0.0 up comes back negative and prints as nonsense. Every value here is pushed back through an unsigned shift. If a subnet calculator gives you sensible answers for 10.x and garbage for 200.x, that is the bug.

The wildcard mask — the inverted subnet mask — is worth knowing because Cisco access lists and OSPF use it instead of a subnet mask, and reading one as the other silently matches the wrong addresses.