Classful routing protocols determine the subnet mask of a network via the first octet of an IP-Address. They do not send any subnet mask information to other routers. Thus, subnet masks must be conform with their class-bound IP Addresses.

 Classless routing protocols send subnet mask information to other routers. Any IP/subnet mask combination is possible with classless routing.

 IPv4 was a classful protocol at first. This changed when the amount of available IP Addresses for internet connection turned out to be too low. The class limitation was removed to increase the available combinations of IP addresses and subnet masks. This also required new routing protocols to handle the now classless IP addresses.

 IPv6 was never classful. In fact, the amount of available IP addresses with IPv6 is so high, that every device can have its own, unique IP address. This creates the possibility of a single, global network where no IP address is used twice. Subnetting would become an optional feature.

RIPv1 (Routing Information Protocol version 1) and IGRP (Interior Gateway Routing Protocol) are classful routing protocols used to dynamically share routing information between routers.

Figure 1:

R1 sends its routing information to R2.

 Figure 2:

R2 receives routing information from R1 and sets the subnet mask of R1’s routes according to their class.

 Figure 3:

R3 sends routing information to R2.

 Figure 4:

R2 receives routing information from R3. R2 now believes that R3 can be accessed via the same port as R1 and vice versa, since both routers offered the same network address to R2.

 Figure 5: The ping sent out by R2 is always sent out on the wrong port, since R1 (the target device) never answers.

 The tracert command shows erratic behaviour.  It never reaches its destination, as each tracers package is sent out on a different port.