Understanding and configuring classless IP addressing and use of IP classless command on Cisco router
Classless IP addressing means using the IP address of one class with the subnet mask of another class.
For example, 10.1.1.1 is the default class A IP address with the default subnet mask of 255.255.255.0 however if we configure other subnet masks like 255.255.255.0 then it becomes a classless IP address as we have changed the default subnet mask of 10.1.1.1 IP address.
Classless IP addressing is common when we segment the bigger networks into smaller ones using the subnetting for example default class a subnet mask provides 16,777,214 valid host IPs in one network which is too much because as per network design recommendation, we should not have more than 500 devices in one network as it will slow down the network.
Class C subnet mask will only provide 254 hosts IP in one network so we use subnetting and use classless IP addresses while creating a network.
Class C subnetting is also very common so we will see classless IP addressing within the class C IP range like if we are using 255.255.255.128 subnet mask then it becomes the classless IP addressing.
Classless vs classful routing
Older routing protocols like IGRP and RIP are classful routing protocols however newer routing protocols like OSPF and EIGRP are classless routing protocols which means that RIP cannot recognize the mismatch between when we use classless IP addressing so it does not matter which IP address we use, RIP protocol will only consider the classful IP address however classless routing protocols automatically recognize the classless IP addressing when we use different subnet mask.
Not supporting classless IP addressing is a big drawback as subnetting is used in most of the networks and we cannot create proper routes if our routing protocol does not support the classless IPs.
Classless IP command
RIP routing protocol doesn’t support the classless IP addressing by default but we can use the command IP classless to make RIP protocol work with the classless IP configuration.
EIGRP automatically learned without typing any command because it supports the classless IP addressing by default.
