4.2 How Routers Determine the Routes

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While the idea of the collapsing many IP addresses into a single network number greatly reduces the number of individual endpoints that a router must track to properly route packets, each router still needs a way to learn the path from itself to each of the network numbers it might encounter.

When a new core router is connected to the Internet, it does not know all the routes. It may know a few preconfigured routes, but to build a picture of how to route packets it must discover routes as it encounters packets. When a router encounters a packet that it does not already know how to route, it queries the routers that are its “neighbors”. The neighboring routers that know how to route the network number send their data back to the requesting router. Sometimes the neighboring routers need to ask their neighbors and so on until the route is actually found and sent back to the requesting router.

In the simplest case, a new core router can be connected to the Internet and slowly build a map of network numbers to outbound links so it can properly route packets based on the IP address for each incoming packet. We call this mapping of network numbers to outbound links the “routing table” for a particular router.

When the Internet is running normally, each router has a relatively complete routing table and rarely encounters a new network number. Once a router figures out the route to a new network number the first time it sees a packet destined for that network number, it does not need to rediscover the route for the network number unless something changes or goes wrong. This means that the router does a lookup on the first packet, but then it could route the next billion packets to that network number just by using the information it already has in its routing tables.

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