Pure bit arithmetic: no network calls, no database. Address and mask are computed on the server and returned to this page once.
A mask must be a run of consecutive 1s from the most significant bit. Non-contiguous masks such as 255.0.255.0 are invalid per RFC 950 — this tool reports an error instead of guessing.
IPv4 only. IPv6 prefixes are 128 bits wide and have no broadcast address, so the semantics differ and are out of scope here.