The Routing Information Protocol version 1 (RIPv1) is a classful routing protocol, which exchanges routes without including any subnet masking information. IP addresses in the routing table should have the same subnet mask. Because classful routing protocols may not fully utilize the available IP address range, all router interfaces within the same network must have the same subnet mask.

Open Shortest Path First (OSPF), Routing Information Protocol version 2 (RIPv2), Enhanced Interior Gateway Routing Protocol (EIGRP), and Border Gateway Protocol version 4 (BGPv4) are classless routing protocols. These protocols include the subnet mask in the route advertisement and support variable length subnet masks (VLSM). Intermediate System-to-Intermediate System (IS-IS) is also a classless routing protocol. An example of a network using VLSM is shown below.
Note the different masks used, indicated with CIDR notation.