| 7.2 |  | RIP |
| 7.2.3 |  | Using the ip classless command |

| Sometimes a router receives packets destined for an unknown subnet of a network that has directly connected subnets. In order for the Cisco IOS software to forward these packets to the best supernet route possible, use the ip classless global configuration command. A supernet route is a route that covers a greater range of subnets with a single entry. For example, an enterprise uses the entire subnet 10.10.0.0 /16, then a supernet route for 10.10.10.0 /24 would be 10.10.0.0 /16. The ip classless command is enabled by default in Cisco IOS Software Release 11.3 and later. To disable this feature, use the no form of this command. When this feature is disabled any packets received that are destined for a subnet that numerically falls within the router’s subnetwork addressing scheme will be discarded. IP classless only affects the operation of the forwarding processes in IOS. IP classless does not affect the way the routing table is built. This is the essence of classful routing. If one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped. The most confusing aspect of this rule is that the router only uses the default route if the major network destination does not exist in the routing table. A router by default assumes that all subnets of a directly connected network should be present in the routing table. If a packet is received with an unknown destination address within an unknown subnet of a directly attached network, the router assumes that the subnet does not exist. So the router will drop the packet even if there is a default route. Configuring ip classless on the router resolves this problem by allowing the router to ignore the classful boundaries of the networks in its routing table and simply route to the default route.
| Lab Activity e-Lab Activity: Using Classless IP Routing In this lab, the student will configure the router to use classless routing when forwarding packets.
| Web Links ip classless Global Configuration Command http://www.cisco.com/en/US/products/sw/ iosswrel/ps1835/ products_command_reference_ chapter09186a0080087387.html#1018036