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CIDR – Classless Inter Domain Routing

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.

The CIDR value is a value that indicates how many bits does the Subnet mask have turned “on”

 For example, if I have the IP address with a Subnet mask of, 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

 The ‘/8’ tells us there are 8 bits used in the subnet mask.


So if we had an IP address of with a subnet mask of (In binary 11111111.11111111.00000000.00000000), there are 16 bits turned ‘on’ so the short version would be


And could be written as

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