In Figure 3-7, Barney
and Fred are both at their homes, and they are excited about looking for the
latest in bowling balls from Fred's Company website (www.fredsco.com). So, from
their home computers, they each have a browser window open looking at
www.fredsco.com.
So that both Fred and Barney can see their web pages, the web
server returns packets to both of them. Packets sent to Fred have Fred's IP
address in the IP header, and packets sent to
Barney have his IP address in the header, as shown in the figure. (Headers are
extra bytes that are sent with a packet.) A protocol uses those extra bytes to
hold information that the protocol needs to do its job, like the IP addresses
inside IP headers.
ISP3's network includes a lot of devices and cables, including
routers. (The router icon looks
like a hockey puck with arrowed lines on top in the figure.) When the packets
sent by the web server get to ISP3's router, the router looks at the IP address
and decides where to forward the packet. Packets sent to Fred go to ISP1, and
packets sent to Barney go to ISP2.
To know where to forward the packets, the router uses a table,
cleverly called the routing table. The routing table
works like a road sign by the highway: It tells the traffic which turn to take
to reach the right destination.
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