CIDR Calculator

Classless Inter-Domain Routing, usually shortened to CIDR, represents an IPv4 network with an address followed by a slash and prefix length. A value such as 192.168.1.42/24 says that the first twenty-four bits identify the network and the remaining eight bits identify addresses inside it. This CIDR calculator accepts the address and prefix separately, then displays the boundaries and masks needed for network planning, firewall rules, routing tables, and troubleshooting. An IPv4 address contains thirty-two binary bits arranged as four decimal octets. The prefix determines how many leading bits are fixed. The calculator creates a subnet mask with that number of leading ones, performs a bitwise AND with the supplied address to find the network address, and sets every host bit to one to find the broadcast address. The wildcard mask is the bitwise inverse of the subnet mask and is commonly used by access-control lists and routing configurations. Total address capacity is two raised to the number of host bits. In a conventional subnet, the all-zero host address identifies the network and the all-one host address is the broadcast, leaving two fewer assignable host addresses. Prefixes /31 and /32 are special. A /31 is widely used for point-to-point links under RFC 3021, where both addresses can represent endpoints, while a /32 identifies one host route. The calculator therefore reports both addresses as usable for /31 and one for /32 rather than subtracting two. The usable range is shown from the first host through the last host. For ordinary prefixes, those are one address above the network and one below the broadcast. Remember that the displayed capacity describes address mathematics, not necessarily the number of devices your environment can support. Gateways, infrastructure, reserved addresses, DHCP exclusions, and cloud-provider policies can reduce practical capacity. IPv6 also uses CIDR prefixes, but its 128-bit addresses and notation require a dedicated IPv6 calculator; this tool intentionally validates IPv4 only. CIDR notation replaced rigid class A, B, and C network boundaries, allowing administrators to allocate address space in blocks that better match demand. Moving the prefix one bit higher halves the block, while moving it one bit lower doubles it. For example, a /24 has 256 total addresses and a /25 has 128. Use the calculator when dividing a larger allocation, checking whether two hosts share a subnet, documenting VPN routes, or translating an unfamiliar prefix into a dotted-decimal mask. Always confirm production changes against your organization's address plan before applying them.