Classful routing is used in routing packets based upon the class of IP address. IP addresses are divided into
five classes: Class A, Class B, Class C, Class D, and Class E. Class A, Class B and Class C are used to
private and public network addressing; Class D is used for multicast broadcasting; and Class E is reserved
by the Internet Assigned Numbers Authority (IANA) for future use.
Classful routing is a consequence of the fact that routing masks are not advertised in the periodic, routine,
routing advertisements generated by distance vector routing protocols. In a classful environment, the
receiving device must know the routing mask associated with any advertised subnets or those subnets cannot
be advertised to it. There are two ways this information can be gained:
• Share the same routing mask as the advertising device
• If the routing mask does not match, this device must summarize the received route a classful boundary
and send the default routing mask in its own advertisements.
Classful routing protocols, such as Routing Information Protocol version 1 (RIPv1) and Interior Gateway
Routing Protocol (IGRP), exchange routes to subnetworks within the same network if network administrator
configured all of the subntworks in the major network have the same routing mask. When routes are
exchanged with foreign networks, subnetwork information from this network cannot be included because the
routing mask of the other network is not known. As a result, the subnetwork information from this network
must be summarized to a classful boundary using a default routing mask prior to inclusion in the routing
update. The creation of a classful summary route at major network boundaries is handled automatically by
classful routing protocols. However, summarization at other points within the major network address is not
allowed by classful routing protocols.