Classful IP routing protocols
These are routing protocols that do not send subnet mask information when a route update is sent out. Examples are; RIP version 1 and IGRP (Interior Gateway Routing Protocol). Classful routing protocols don’t have a field for subnet information, so the subnet information gets dropped. What this means is that if a router running RIP version 1 or IGRP has a subnet mask of a certain value, it assumes that all interfaces within the classful address space have the same subnet mask.
If you mix subnet mask lengths in a network running RIP or IGRP, that network just won’t work!
Classful network designs are inefficient and wasteful of IP addresses because you have to use the same subnet mask everywhere including point to Point interfaces which would only require two IP addresses.
Another disadvantage of classful routing protocols is that they don’t support summarization of routes and hence the routing tables for such networks tend to grow very big which increase the time a router will take to forward a packet and eventually slows down the network.
Very big routing table are also a head ache to manage and troubleshoot.
Classless IP routing protocols
Classless routing protocols, however, do support the advertisement of subnet information. I.e. these routing protocols send subnet mask information in the routing updates.
Examples of classless routing protocols are; RIP version 2, EIGRP and OSPF.
Therefore, you can use Variable Length Subnet Masks (VLSM) with these routing protocols.
VLSM means we can have different subnet masks for different router interfaces.
This technique helps to save IP address space because you can use subnet masks of different lengths on the different interfaces.
Another advantage of classless routing protocols is that they support summarization of IP routes; this keeps the routing tables in such networks small and easy to manage.
Routers will also spend less time doing route lookups in such small routing tables and hence enhance the forwarding speeds.