IP Subnet Calculator

A subnet mask marks how many leading bits of an address name the network; the rest number the hosts inside it. Enter any address in the subnet together with its mask, as a prefix (/24) or dotted ( 255.255.255.0), and the calculator clears the host bits to find the network, sets them all to find the broadcast address, and counts what is left between them.

Usable hosts, and the two exceptions

An IPv4 subnet gives up its first address to the network identifier and its last to broadcast, so a /24 has 256 addresses and 254 usable hosts. That rule breaks twice at the bottom of the table. A /31 has two addresses and both are usable on a point-to-point link (RFC 3021). A /32 is one host. Both are reported as the RFCs define them rather than as 2ⁿ − 2.

Reading the binary view

The bar in each binary row sits where the prefix ends: network bits to its left, host bits to its right. The network address is the input with everything right of the bar set to zero; the broadcast is the same with everything set to one. That is the whole calculation, and seeing it is the fastest way to check an answer by hand.

Splitting a subnet

Equal subnets come in powers of two. Ask for five and you get eight, each a /27 of a /24, because five need three more prefix bits. Splitting by prefix instead gives every subnet of that size. Very large splits are capped at 256 listed rows, with the true total stated alongside.

For IPv6 prefixes, or CIDR blocks written without an address in mind, use the CIDR Calculator. To turn an arbitrary start-end range into blocks, use the CIDR Range Calculator. More utilities are listed on the tools index.

10.20.30.77 255.255.255.192 is in 10.20.30.64/26: network 10.20.30.64, broadcast 10.20.30.127, hosts 10.20.30.65 to 10.20.30.126, 62 usable, wildcard 0.0.0.63. Dividing 192.168.0.0/24 into 4 subnets gives four /26s starting at .0, .64, .128 and .192, with 62 usable hosts each.