Static Default Routes

A default route has all 0s as its destination IP address. The default route can be automatically generated by a routing protocol or manually configured. The manually configured default route simplifies network configuration. If the destination address of a packet fails to match any entry in the routing table, the router selects the default route to forward the packet.

Figure 2-6 Networking diagram of static routes

In Figure 2-6, if no static default route is configured, you need to configure static routes destined for networks 3, 4, and 5 on RouterA, configure static routes destined for networks 1 and 5 on RouterB, and configure static routes destined for networks 1, 2, and 3 on RouterC. In this way, RouterA, RouterB, and RouterC can communicate with each other.

The next hop of the packets sent by RouterA to networks 3, 4, and 5 is RouterB. Therefore, a default route configured on RouterA can replace the three static routes destined for networks 3, 4, and 5 in the preceding example. Similarly, just one default route from RouterC to RouterB can replace the three static routes destined for networks 1, 2, and 3 in the preceding example.