Subnet Calculator

The Subnet Calculator takes an IPv4 address and a subnet definition and works out everything you need to plan or document a network.

Give it CIDR notation such as 192.168.1.10/24, or type an address and supply the mask separately — either as a prefix length like 24 or a dotted-decimal mask like 255.255.255.0 — and it returns the network (base) address, the broadcast address, the first and last usable host, the count of usable hosts, the total address block size, the wildcard (inverse) mask and the mask in binary.

It also classifies the address: the legacy class (A through E), and whether it falls inside the RFC 1918 private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).

Edge cases are handled correctly, including /31 point-to-point links where both addresses are usable and /32 single-host routes, so the host counts stay accurate rather than silently subtracting two.

This is the everyday tool for sizing VLANs, carving a block into smaller subnets, sanity-checking a firewall rule or a router config, and answering "how many hosts fit in a /26?" without reaching for a whiteboard.

Because all of the arithmetic is plain bit manipulation, it runs entirely in your browser with nothing sent to a server, so it is fast, private and works offline — useful when you are deep in a data centre with no connection.