??? To do so, the client must identify the
server, and must know in advance the port on which the server is ???listening.???
Since each computer has thousands of ports available, you may be wondering how the typical client
application could possibly guess on which port to call the server? The answer is that the use of a small number
of ports is standardized, and those port numbers are well known. For instance, web servers listen on port 80, so
all of those visits you make to websites are really contacts with web server programs listening on port 80 of the
web server computer.
After the client contacts the server, the client waits for an acknowledgement from the server of the client??™s
request to open a connection. When the client receives one, the client acknowledges back to the server that the
client has received the server??™s acknowledgement. At this time, both computers have established a connection
on which to communicate.
This three-step connection establishment protocol may remind you of how people use telephones. We dial
the other person (the server, or object of connection), we hear the other person pick up the phone and say,
???Hello,??? (akin to the server??™s acknowledgement of the client??™s open request), and we identify ourselves in
confirmation, ???Hi.
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