CIDR Calculator

Classless Inter-Domain Routing (CIDR) replaces traditional network classes with a flexible suffix indicating the number of bits allocated to the network portion of an IP address. The notation consists of an IP address followed by a slash and a decimal number between 0 and 32 (e.g., /24). The remaining bits define the host space. The total number of IP addresses in a block is calculated using the formula 2^(32 - n), where n represents the CIDR prefix length. From this total range, the first address serves as the network identifier and the last address acts as the broadcast address, leaving 2^(32 - n) - 2 usable host IP addresses for network devices.

Consider a network assigned the CIDR block 192.168.10.0/24. To find the network parameters, we first determine the prefix length (n = 24). The total number of addresses is 2^(32 - 24) = 2^8 = 256. The subnet mask is derived by turning on the first 24 bits, yielding 255.255.255.0. The wildcard mask, which is the inverse of the subnet mask, is 0.0.0.255. The network address is 192.168.10.0, and the broadcast address is 192.168.10.255. Subtracting the network and broadcast IDs leaves a usable host range from 192.168.10.1 to 192.168.10.254, providing 254 usable host addresses.

When planning your network architecture, always account for future expansion by selecting a slightly larger CIDR block than your current device count requires. Double-check your wildcard masks when configuring access control lists (ACLs) or routing protocols like OSPF, as wildcard bits are the exact logical inverse of standard subnet masks. Keep a clear documentation registry of your network boundaries to prevent overlapping IP allocations across different virtual local area networks (VLANs).