Classfull: It does not send subnet mask with it.
Summarization is done at the classfull boundary.
It may lead to suboptimal path forwarding.
Classless: It sent the subnet mask with it.
Classfull address: The address with default mask e.g. 10.0.0./8
Classless address: The address with any subnet mask other than default subnet mask.
Classfull Routing Protocol: suppose we have three router connected to each other and RIPv1 or IGRP(Interior Gateway Routing Protocol) are configured on all routers. Router 2 send the network without subnet mask also router 3 send without subnet mask, but router cannot accept the network without subnet mask. So it check the network bits and add default mask according to the address belong to which class. For example. If a address is class A, then router add default mask ‘/8’ and add it in routing table. If someone ping 10.20.20.1 to reach the destination, but there is no way to reach on destination. Router has two path to reach the network, both are same. In this condition, the router will sent packet to both direction. One will accept it (who request to ping) and the other will decline it.
Note: Classfull routing protocols are protocol in which:
Mask is not sent as part of update.
They will automatically summarize at classfull boundary.
It may lead to suboptimal path forwarding.
