Subnet Calculator (IPv4 CIDR)

A subnet mask defines which part of an IP address is the network and which part identifies individual hosts. CIDR notation expresses this as a slash followed by the number of network bits, so /24 equals the mask 255.255.255.0 and covers 256 total addresses. The calculator also shows the wildcard mask, which is the bitwise inverse of the subnet mask and is used in firewall and ACL rules.

A /24 is common and provides 254 conventional host addresses, but it is not required and may be too small or larger than needed. Consumer-router defaults often use private ranges such as 192.168.1.0/24. Use a prefix that fits the network design and avoids overlap.

Every subnet reserves its first address as the network identifier and its last as the broadcast address. A /24 has 256 total addresses (2^8) but only 254 are assignable to devices. The exceptions are /31 and /32: a /31 treats both addresses as usable hosts (per RFC 3021), and a /32 represents a single host with 1 usable address.

Classful allocation used default /8, /16, and /24 network boundaries. CIDR allows any prefix length from /0 to /32 and therefore offers more granular power-of-two blocks. It reduces waste, but it cannot allocate every arbitrary host count exactly because network sizes remain powers of two.

A /32 identifies a single host and appears in routing tables, loopback addresses, and firewall rules targeting one IP. The calculator shows 1 usable address for /32. A /31 (RFC 3021) is designed for point-to-point links between two routers, where both addresses are usable so no addresses are wasted on a network or broadcast identifier.