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Any server machine makes its services available to the Internet using numbered ports,
one for each service that is available on the server. For example, if a server
machine is running a Web server and an FTP server, the Web server would typically
be available on port 80, and the FTP server would be available on port 21. Clients
connect to a service at a specific IP address and on a specific port.
Each of the most well-known services
is available at a well-known port number. Here are some common
port numbers:
- echo 7
- daytime 13
- qotd 17 (Quote of the Day)
- ftp 21
- telnet 23
- smtp 25 (Simple Mail Transfer,
meaning e-mail)
- time 37
- nameserver 42
- nicname 43 (Who Is)
- gopher 70
- finger 79
- WWW 80
If the server machine accepts connections on a port from the
outside world, and if a firewall is
not protecting the port, you can connect to the port from anywhere
on the Internet and use the service. Note that there is nothing
that forces, for example, a Web server to be on port 80. If you
were to set up your own machine and load Web server software
on it, you could put the Web server on port 918, or any other
unused port, if you wanted to. Then, if your machine were known
as xxx.yyy.com, someone on the Internet could connect to your
server with the URL http://xxx.yyy.com:918.
The ":918" explicitly specifies the port number, and would have
to be included for someone to reach your server. When no port
is specified, the browser simply assumes that the server is using
the well-known port 80. |
