CIDR was invented in 1990, and was the change that allowed routers to use classless addresses to route traffic instead of class-full addresses. This helped a huge amount as before its invention, routers could only route traffic based on full classes. If we look at Class A address for example, there are only 126 of them (1-126 in the first octet), so before CIDR only 126 companies could have a Class A address. It also posed a problem, since a single Class A address gave over 1.6 million IP addresses, so when companies registered a full Class A, there was a lot IP addresses that went unused. By allowing us to ‘slice and dice’ a Class A into smaller networks, we were able to save a huge amount of IP addresses, and of course give us much finer granularity.
For example, if I have the IP address 10.1.2.3 with a Subnet mask of 255.0.0.0, and we convert the subnet mask into binary we get 11111111.00000000.00000000.00000000 – we can see clearly that there are 8 bits turned ‘on’ in the subnet mask.
Therefore we could write the short version of our IP address as 10.1.2.3/8
The ‘/8’ tells us there are 8 bits used in the subnet mask.
So if we had an IP address of 172.16.1.2 with a subnet mask of 255.255.0.0 (In binary 11111111.11111111.00000000.00000000), there are 16 bits turned ‘on’ so the short version would be 172.16.1.2/16
And 192.168.1.10 255.255.255.0 could be written as 192.168.1.10/24