Classless Routing
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).