Cisco Systems 78-11741-02 manual Disabling Classless Routing Behavior, IPC-10

Configuring IP Addressing

Assigning IP Addresses to Network Interfaces

You can use the all 0s and all 1s subnet (131.108.255.0), even though it is discouraged. Configuring interfaces for the all 1s subnet is explicitly allowed. However, if you need the entire subnet space for your IP address, use the following command in global configuration mode to enable subnet 0:

Command	Purpose
Router(config)# ip subnet-zero	Enables the use of subnet zero for interface addresses and routing
	updates.

Disabling Classless Routing Behavior

By default, classless routing behavior is enabled on the router. When classless routing is in effect, if a router receives packets destined for a subnet of a network that has no network default route, the router forwards the packet to the best supernet route.

In Figure 1, classless routing is enabled in the router. Therefore, when the host sends a packet to 128.20.4.1, instead of discarding the packet, the router forwards the packet to the best supernet route.

Figure 1 IP Classless Routing

128.0.0.0/8

128.20.4.1

128.20.0.0 ip classless

128.20.1.0

128.20.2.0

128.20.3.0

128.20.4.1

S3286

Host

If you disable classless routing, and a router receives packets destined for a subnet of a network that has no network default route, the router discards the packet. Figure 2 shows a router in network 128.20.0.0 connected to subnets 128.20.1.0, 128.20.2.0, and 128.20.3.0. Suppose the host sends a packet to 128.20.4.1. Because there is no network default route, the router discards the packet.

Cisco IOS IP Configuration Guide

IPC-10