In the earlier example, we showed Netscape sending a TCP request
to a web server running on another node. But imagine that the
node with the web server is also running an Email server, an FTP
server and a name server: how does it know which server the TCP
connection is for? This is where TCP and UDP have a concept of
`ports'. Every packet has space for a `destination port', which
says what service the packet is for. For example, TCP port 25 is
the mail server, and TCP port 80 is the web server (although
sometimes you find web servers on different ports). A list of
ports can be found in `/etc/services'. Also, if two Netscape
windows are both accessing different parts of the same web site,
how does the Linux box running Netscape sort out the TCP packets
coming back from the web server? This is where the `source port'
comes in: every new TCP connection gets a different source port,
so everyone can tell them apart, even if they are going to the
same destination IP address and the same destination port.
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