What does the number after a slash mean on an IP address? What is a netmask?



An IP address consists of four octets, each consisting of an eight digit binary number.

00000000 through 11111111

In decimal this would be: 0 through 255
 

So an IP address consists of four eight digit binary numbers. The total number of digits in an IP address would be 32.

The netmask is used to identify which digits in an IP address are not variable.

You’ll also see an indication of this by putting a “/24”, for example, at the end of an IP address.

A netmask of 255.255.255.0 is an indication that the entire first three octets (i.e. the first 24 binary digits) are set. The entire last octet is variable.

An IP address with a netmask of 255.255.255.0 is the same as seeing that IP with a “/24” at the end of it.
 

The “/24” indication is called CIDR. (classless, inter-domain routing)

It was created because we were running out of IP addresses to use.

The “/24” CIDR notation means the last octet is variable. This is also known as a class C network.

129.*.*.*             Class A network

129.100.*.*         Class B network

129.100.100.*    Class C network

All IP address ranges were broken up in this manner for many years. It created a lot of wasted, unused IP addresses for networks that were not "just the right size".

CIDR was developed so you could break an IP address down at any point in the 32 binary digits, hence “classless”.


Example:

A “/23” network means that the first 23 binary digits are set, the rest are variable.

192.100.100.0/24 indicates:

192.100.100.0     through 192.100.100.255

192.100.100.0/23 indicates:

192.100.100.0     through 192.100.101.255

That “/23” means that this IP range consists of two class C networks and not just one.