IPv4 subnet cheat sheet

How to read the table

The CIDR figure counts the bits that identify the network. A /24 fixes the first 24 bits and leaves 8 for hosts, which is why it holds 256 addresses. Every step up the table doubles the block: a /23 is two /24s, a /22 is four.

The subnet mask is the same information written the older way, as four octets. The wildcard mask is that inverted, which is the form access lists on most routing platforms expect. It causes a lot of configuration mistakes, because the two look similar and mean the opposite.

Why usable hosts is two fewer than the address count

The first address in a block names the network itself and the last is the broadcast address, so neither can be assigned to a machine. A /24 holds 256 addresses and numbers 254 hosts.

Two prefixes break that rule, and both appear in real configurations. A /31 has no room for a network and broadcast address, so RFC 3021 defines both of its addresses as usable, which is the standard way to number a point-to-point link. A /32 is a single address with no network to broadcast to, which is how loopbacks and single-host routes are written.

Class A, B and C

Classful addressing was replaced by CIDR in 1993, but the vocabulary outlived it. Registries, brokers and buyers still say "a Class C" when they mean a /24. The last column translates, because the terms are still what people negotiate in, not because the classes still exist.

Common questions

How many IP addresses are in a /24?

256 addresses, of which 254 can be assigned to hosts.

What is the subnet mask for a /22?

255.255.252.0, covering 1,024 addresses and 1,022 usable hosts.

What is the smallest block that can be routed on the internet?

A /24. Longer prefixes are filtered by most networks, so a /25 or smaller can be used inside your own network but cannot be announced globally on its own.