One of the most serious limitations in a classful network environment is that the routing mask is not
exchanged during the routing update process. This requires the same routing mask be used on all
subnetworks. The classless approach advertises the routing mask for each route and therefore a more precise
lookup can be performed in the routing table. Classless routing, which is also known as Classless
Interdomain Routing (CIDR), is thus not dependent on IP address classes but, instead, allows a variablelength
subnet mask (VLSM), which extends IP addressing beyond the limitations of using fixed-length
subnet masks (FLSM),to be sent in the routing update with the route. This allows you to conserve IP
addresses, extending the use of IP addresses. Classless routing protocols also addressed the need to
summarize to a classful network with a default routing mask at major network boundaries. In the classless
environment, the summarization process is manually controlled and can be invoked at any point within the
network.
The routing protocols that support classless routing protocols are: Routing Information Protocol version 2
(RIPv2); Enhanced Interior Gateway Routing Protocol (EIGRP); Open Shortest Path First (OSPF); and
Integrated Intermediate System to Intermediate System (IS-IS).