ip classless

All Cisco routers are classful routers, meaning they expect a default subnet mask on each interface of the router. When a router receives a packet for a destination subnet that’s not in the routing table, it will drop the packet by default. If you’re using default routing, you must use the ip classless command because it is possible that no remote subnets will be in the routing table.

 

Router(config)# ip route 0.0.0.0 0.0.0.0 下一跳路由器的IP地址或者 退出接口

Router(config)# ip classless

 

Notice that it's a global configuration mode command.