IP Subnet Calculator

What the prefix length actually means

An IPv4 address is 32 bits. The prefix length says how many of those leading bits identify the network; the rest identify the host inside it. A /24 fixes the first 24 bits, leaving 8 host bits, so the subnet contains 2⁸ = 256 addresses. The binary view above shows the boundary directly, which is usually faster than reasoning about the dotted-decimal mask.

Why usable hosts is two fewer than total addresses

The all-zeros host address names the network itself and the all-ones host address is the broadcast address; neither can be assigned to a machine. So a /24 has 256 addresses but 254 usable ones. The exceptions are /31 and /32: RFC 3021 allows a /31 to be used for a point-to-point link with both addresses assigned, and a /32 is a single host route.

Private ranges, and the one that keeps surprising people

RFC 1918 reserves 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 for private use. The middle one catches people out: it is 172.16 through 172.31, not all of 172.x. There is also 100.64.0.0/10 (RFC 6598) for carrier-grade NAT, 169.254.0.0/16 for link-local autoconfiguration — the address your machine assigns itself when DHCP fails — and 127.0.0.0/8 for loopback.

Classes are history, but the vocabulary survives

Classful addressing was replaced by CIDR in 1993. The old class letters are shown above because documentation and interview questions still use them, not because anything routes that way now. “A class C network” is a phrase people use to mean /24; the two have not formally been the same thing for three decades.

Frequently Asked Questions