Subnet Calculator (IPv4 / CIDR)

Give it an IPv4 address with CIDR (like 192.168.1.0/24) and it returns the network address, broadcast, netmask, wildcard mask, first/last usable host, host count, and IP class. Also detects RFC 1918 private ranges (10/8, 172.16/12, 192.168/16), loopback (127/8), and link-local (169.254/16). Includes a binary view so you can see how the mask aligns. Handles /31 (point-to-point) and /32 (single host) edge cases correctly.

The usable host count is two fewer than the address count because the first and last are reserved: the all-zeros host identifies the network itself and the all-ones host is the broadcast. That is why a /24 offers 254 addresses rather than 256, and why the arithmetic bites hardest at the small end — a /30 has four addresses and exactly two usable, which is the classic four-address block for a router link.

The two exceptions are the ones worth knowing. RFC 3021 lets a /31 carry two usable hosts on a point-to-point link, where broadcast has no meaning, so the usual subtraction would leave zero and the rule is suspended. A /32 is a single address with no network or broadcast at all, used for loopback interfaces and host routes. A calculator that applies the minus two blindly reports nonsense for both.

The binary view is there because masks are easier to see than to reason about. A prefix length is a count of leading ones, so a /26 mask is 11111111.11111111.11111111.11000000 and the boundary sits mid-octet at 192. That is where subnetting stops being decimal arithmetic and starts being bit alignment, and it is why /25, /26 and /27 boundaries land on 128, 64 and 32 rather than anywhere round.

Address classes are shown for reference and have not governed routing since CIDR arrived in 1993. A modern network is defined by its prefix length, and the class of an address tells you nothing about how it is actually routed.