CIDRKit

Subnet and CIDR arithmetic: network address, netmask, wildcard mask, broadcast, the usable range, both counts, and range classification for IPv4. Accepts a prefix, a dotted mask, a Cisco wildcard mask or an inclusive address range, and it reads IPv6 exactly — including counts up to 2^128, which is why the engine works in BigInt rather than numbers.

It also does the two jobs that are annoying by hand: collapsing a pasted list of CIDR blocks into the smallest equivalent set (50,000 entries in 187.7 ms), and splitting one block into equal smaller subnets with the per-subnet host count spelled out, including the /31 and /32 cases where the usual minus-two rule is wrong.

Every number this site prints is checked against Python's standard-library ipaddress module — 356 valid inputs compared field by field, 15 malformed inputs that must be rejected, and 20 capability cases for collapse, split and containment. Nothing you paste is uploaded; there is no server call anywhere in the code path.