IP Subnet Calculator

CIDR was standardized in 1993 to replace classful routing and allow arbitrary IPv4 prefix lengths from /0 to /32. Classful networks used fixed default masks for Classes A, B, and C, but subnetting could already create longer prefixes inside them. In CIDR, /N is the number of leading network bits: /24 is 255.255.255.0 and /22 is 255.255.252.0.

The subnet mask is a 32-bit value with the leading prefix bits set to 1 and the rest set to 0. To find the network address you AND the IP with the mask: 192.168.1.42 AND 255.255.255.0 keeps the first three octets and zeros the last, giving 192.168.1.0. To find the broadcast you OR the network with the wildcard mask (the bitwise inverse): 192.168.1.0 OR 0.0.0.255 gives 192.168.1.255. The number of total addresses in the block is 2 to the power of the host-bit count, so a /24 has 2^8 = 256 addresses and a /22 has 2^10 = 1,024.

RFC 1918 (1996) carved out three ranges for use inside private networks, with the promise that the public internet routing system will never carry them: 10.0.0.0/8 (one Class A block, 16,777,216 addresses), 172.16.0.0/12 (sixteen contiguous Class B blocks, from 172.16.0.0 to 172.31.255.255, just over a million addresses), and 192.168.0.0/16 (256 contiguous Class C blocks, 65,536 addresses). Consumer routers almost always pick a /24 from inside 192.168.0.0/16 or 10.0.0.0/8 for the home LAN. The calculator flags any address that falls inside one of these blocks as private.

RFC 3021 permits both addresses in a /31 on a point-to-point link. A /32 identifies one address, commonly as a host route, with no separate network or broadcast address. This calculator follows that host-route convention and reports one usable host for /32.

IANA allocated its remaining general IPv4 pool to the regional registries in 2011, and regional scarcity followed. Networks now use transfers, address sharing, and IPv6 during the transition. IPv6 uses 128-bit addresses, while this calculator handles IPv4 only.