15.5 ROUTING THROUGH A NETWORK
Once packets have been assigned logical IP addresses, routers must determine the path they should follow through the internetwork. This process is known as routing. Every router makes an independent forwarding decision based on the destination IP address and the routing information available to it. Although the Internet consists of millions of interconnected networks, this simple process allows packets to be delivered efficiently across the world.
15.5.1 Packet Forwarding In Perspective
Every router maintains a routing table that identifies the networks it can reach and the next router, or next hop, that should receive packets destined for each network.
Rather than storing a route to every individual computer, routers maintain routes to networks. This hierarchical approach greatly reduces the amount of routing information that must be stored and exchanged.
A typical routing table contains information such as:
- the destination network;
- the network prefix;
- the next-hop router;
- the outgoing interface; and
- a routing metric indicating the preferred path.
When a packet arrives, the router compares the destination IP address with the entries in its routing table and selects the most appropriate route.
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