Subnet Calculator

The Subnet Calculator takes an IP address and a subnet mask or CIDR prefix and computes the full set of network facts you need to plan or troubleshoot an IP network. From an input like 192.168.1.0/24 it derives the network address, broadcast address, usable host range, total and usable host counts, the wildcard mask, and the dotted-decimal mask. This removes the error-prone binary math of ANDing addresses against masks by hand.

Subnetting works by dividing the 32 bits of an IPv4 address (or 128 bits of IPv6) into a network portion and a host portion. The CIDR prefix, such as /24, states how many leading bits identify the network, leaving the rest for hosts; a /24 leaves 8 host bits, which yields 256 total addresses and 254 usable ones after reserving the network and broadcast addresses. Shortening the prefix to /23 doubles the address space, while lengthening it to /25 halves it.

Network engineers use this to design address plans, size subnets to a department or VLAN, verify that two hosts share a subnet, allocate point-to-point links efficiently with /30 or /31 masks, and check firewall and ACL rules. It is also a quick sanity check when a device cannot reach its gateway because its mask is wrong.

A practical tip: remember that the first address in a subnet is the network identifier and the last is the broadcast, so neither is normally assigned to a host. When you need to think about contiguous blocks rather than a single subnet, the IP Range Calculator is the companion tool, and CIDR aggregation lets you summarize several adjacent subnets into one shorter prefix.