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 53
- 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.
