Classless Inter-Domain Routing (CIDR)

In the 1990s it became apparent that the exhaustion of the IPv4 addresses would be reached in a few years with the ever-growing expansion of Internet technology. The move was made away from assigning IP addresses based on classes to a method that uses ranges of address. IPv4 addresses are now specified using the CIDR notation that specifies subnet masks. The CIDR notation uses the format address/prefix.

The prefix designates the number of bits that will be used by the subnet mask. Let’s say we have a range of IP addresses consisting of and a subnet mask of We would note this address using CIDR as:


Using the CIDR notation allows for custom subnet masks to be created without tying them to the limitations of classes.

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