Hello Khan,

in the old days of classful routing protocols discontigous major networks were not supported.

A major network is a Class A, B, C depending on the value of the first most significant byte:

10/8 is Class A network

172.16.0.0/16 is a Class B

192.168.2.0/24 is a Class C

A major network is partitioned if between two subsets of subnets there is a link but that link has an ip subnet taken from a different major network

example:

net 10/8 with a link in the middle belonging to 192.168.2.0/24

some 10 subnets --- R1 -- 192.168.2.0 --- R2 other 10 subnets

Classful routing protocols don't carry subnet masks and automatically summarize at major network boundary

a router connected to net 192.168.2.0/24 different from R1 and r3 thinks that:

net 10/8 is reachable via R1 or R2.

the no ip classless behavior means that a router internal to left cloud of 10 subnets if has to decide where to send packets destined for an unknown 10 subnet will discard the packet even if a default route is installed in the routing table.

the ip classless behaviour allows to use a default or other route to forward traffic for an unknown subnet of a connected major network

ip classless is default since IOS 12.0

Hope to help

Giuseppe