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Classful Routing Protocols and Classless Routing Protocols

Classful Routing Protocols

Classful routing protocols do not send subnet mask information in routing updates. The first routing protocols such as RIP, were classful. This was at a time when network addresses were allocated based on classes, class A, B, or C. A routing protocol did not need to include the subnet mask in the routing update because the network mask could be determined based on the first octet of the network address.

Classful routing protocols can still be used in some of today's networks, but because they do not include the subnet mask they cannot be used in all situations. Classful routing protocols cannot be used when a network is subnetted using more than one subnet mask, in other words classful routing protocols do not support variable length subnet masks (VLSM). There are other limitations to classful routing protocols including their inability to support discontiguous networks.
Classful routing protocols include RIPv1 and IGRP.

Classless Routing Protocols

Classless routing protocols include the subnet mask with the network address in routing updates. Today's networks are no longer allocated based on classes and the subnet mask cannot be determined by the value of the first octet. Classless routing protocols are required in most networks today because of their support for VLSM, discontiguous networks and other features which will be discussed in later chapters.
In the figure, notice that the classless version of the network is using both /30 and /27 subnet masks in the same topology. Also notice that this topology is using a discontiguous design..
Classless routing protocols are RIPv2, EIGRP, OSPF, IS-IS, BGP.