CIDR / Subnet Calculator

The CIDR / Subnet Calculator takes an IPv4 block in CIDR notation — for example 192.168.1.0/24 — and works out everything you need to plan or document a subnet: the network (base) address, the broadcast address, the first and last usable host addresses, the number of usable hosts, the dotted-decimal subnet mask, and the prefix length.

CIDR notation packs a network address and its mask into a compact form, but the arithmetic behind it is binary masking that is tedious and error-prone to do in your head.

Misjudging how many hosts a /26 holds, or where one subnet ends and the next begins, leads to overlapping ranges and addresses that cannot route.

This tool applies the mask precisely, so an address like 192.168.1.130/24 is snapped down to its true network base of 192.168.1.0 and the broadcast and host range follow correctly.

It also handles the special small blocks: a /31 point-to-point link treats both addresses as usable, and a /32 describes a single host.

Every value comes with a copy button for pasting into firewall rules, DHCP scopes or documentation.

Each octet is validated to 0–255 and the prefix to 0–32, and the whole calculation runs locally in your browser.