All network transactions happen between clients and servers. In most protocols , the clients ask a certain address and receive data. The servers watch a port and give information.
To affect a network connection you need to know the host, the port, and the actions allowed on that port. Each port is associated with a service. Each server watches a different port.
Most web servers run on port 80, though it may sometimes be 8080. FTP lives on port 21, and secure shell (SSH) is on 22. For email, POP, SMTP, and IMAP all live on different ports.
You should note that these addresses are the common port numbers for the different services. A network administrator can change them for his or her network. As long as the client asks for the correct service on the right port at the right address, communication will still happen. Google's mail service, for example, does not run on the common port numbers but, because they know how to access their accounts, users can still get their mail.
