CIDR Calculator

An IPv4 address is 32 ones and zeros. The number after the slash says how many of those are fixed for everyone on the network, leaving the rest free to number individual machines.

So /24 fixes 24 bits and leaves 8, which is 256 addresses. /16 fixes fewer bits and leaves more, so it is a bigger network. A bigger number means a smaller network, which is the part that trips everyone up.

Two addresses in every block are reserved. The first names the network itself, and the last is the broadcast address. So a /24 holds 256 addresses but you can only assign 254 of them to machines.

Two exceptions: /31 gives you both addresses, because a cable between two routers needs no broadcast, and /32 is a single machine on its own.

The netmask is the same thing written as four numbers: /24 is 255.255.255.0. The wildcard mask is that flipped — 0.0.0.255. Cisco firewall rules want the flipped version, which is the usual reason to need it.

You can type any address in the block, not just the first. Enter 172.16.5.130/26 and you will get the block starting at 172.16.5.128, because the calculator clears the machine-specific part. That is why the result sometimes shows an address you did not type.