Choosing Which Road to Take at the First Intersection

Step 2: Choosing Which Road to Take at the First Intersection

When you drive from your house in your car, you typically leave your neighborhood going the same general direction until you get to an intersection. Of course, you know how to get where you are going, so you don't need to look at the road signs to choose which way to turn. However, someone who's been visiting you might not know the local roads. When the visitor get to the intersection, he can look at the road signs to decide where to drive.

Comparing networks to roadways, routers are like an intersection; however, the router makes the choice of where to send a packet, rather than a driver choosing where to turn. When a packet arrives at a router, the router looks at the equivalent of a road signsomething called a routing table in networking. The router looks at where the packet wants to go (the destination IP address) as well as at the routing table. By comparing the two, the router can choose where to send the packet next.

The same internetwork is used for the examples throughout this chapter. At this point in the example, an Ethernet frame is arriving on R1's left-side (Ethernet1) interface. The frame contains the IP packet, sent by Hannah (8.1.1.1), with the destination address being the www.example.com web server (130.4.3.3). Figure 11-9 depicts this point in the process.