Subnet Calculator

How this is calculated

An IPv4 address and its subnet mask each have 32 bits. The mask is a run of 1s for the network part followed by 0s for the host part, and the prefix length after the slash counts the 1s: /24 is 255.255.255.0 (RFC 4632). The network address is the IP address with every host bit set to 0 (a bitwise AND with the mask). The broadcast address has every host bit set to 1.

Example: 192.168.1.10/24. The mask keeps the first three numbers, so the network is 192.168.1.0 and the broadcast address 192.168.1.255. The addresses in between, 192.168.1.1 to 192.168.1.254, are 254 usable hosts: 2 to the power of 8 = 256 addresses, minus the network and the broadcast address.

Inside a number it works the same way. A /27 leaves 5 host bits, so each block has 32 addresses. In 10.20.30.77/27, 77 falls in the block from 64 to 95: the network is 10.20.30.64, the broadcast address 10.20.30.95, and the hosts run from 10.20.30.65 to 10.20.30.94, which is 30 usable addresses.

Two prefix lengths are special. A /31 has only two addresses, and on a point-to-point link both are hosts, with no network or broadcast address (RFC 3021). A /32 is a single address, used to name exactly one host, for example in a route or a firewall rule. The wildcard mask is the subnet mask with every bit flipped (/24 gives 0.0.0.255); router access lists use it.

Private and special addresses

RFC 1918 sets aside three blocks for private networks: 10.0.0.0/8, 172.16.0.0/12 (172.16.0.0 to 172.31.255.255) and 192.168.0.0/16. They are used inside homes and companies and are not routed on the public internet. Other blocks have special uses as well. The calculator names the block your address falls in, using the IANA registry of special-purpose addresses and the multicast range from RFC 5771.

The old classes A, B and C, where the first number of the address decided between a /8, /16 or /24 network, are no longer used for routing. CIDR replaced them, so any prefix length can go with any address.