Navigation Basics: Driving to the Right
Destination
When you want to drive somewhere near your house, you get in
your car, drive on the local roads, and get there. When you need to drive a
longer distance, say to another town, you probably start off on those same local
roads but then get on a highway. After you drive far enough to get to that town,
you get off the highway and use the town's local roads to get to where you want
to go.
Now, imagine that you have a visitor staying at your house, and
he wants to drive to that next town. You could tell him to drive down the road
that runs past your house until he reaches the first intersection, and then
follow the road signs. Your friend could simply look at the road sign at that
intersection, which tells him where to turn. If he reads the road sign at each
intersection, he might make a lot of turns, but he will eventually get where he
wants to go, as long as the road signs list the town to which he wants to
drive.
In TCP/IP networks, routers act as intersections, with the
router making the choice about what direction to forward data packets. In a
typical large network, many routers and LAN switches sit between a client PC and
a server. When one computer sends data to another computer, the routers make
decisions about where to forward the data so that it reaches the correct
destination. For example, in Figure 10-1,
a user (Hannah) on a LAN in Mason wants to get a web page from a web server (http://www.cisco.com) on a LAN in Cincinnati. At some point,
Hannah's PC sends the initial HTTP GET request to the web server. The figure
shows that initial request, with the routers deciding where to forward the data
so that it reaches the web server.
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