Classless Inter-Domain Routing and Subnetting

After learning about classful addressing, we know how the IP address blocks have been organized into different ranges within classes, each with a different number of IP addresses. Is it scalable and sustainable to assign IP addresses using classful addressing? Assume we require 400 IP addresses for our office network; which class of IP range should we choose to meet our needs?

We can’t use class C because it only allows for 256 IP addresses in a network; our other options are class A (16777216 IPs) or class B (65536 IPs). Class B appears to be a better option, but it would still be a waste of 65136 (65536 - 400) IP addresses.

This is why traditional classful addressing was not scalable or sustainable, resulting in inefficient routing and address allocation. To address this issue, Classless Inter-Domain Routing (CIDR) was introduced as an extension to classful addressing in 1993. Before diving into CIDR, let’s look at some important concepts.

Subnet mask

In classful addressing, we learned how different classes have different numbers of bits ...