Application-level protocols are responsible for initiating some sort of request (on the client) or answering that request (on the server). When you surf the Internet, for example, you use an application (a web browser) that sends a request to a web server for a specific page. This communication happens across TCP/IP using an Application-layer protocol known as Hypertext Transfer Protocol (HTTP).
A TCP/IP client computer initiates contact with a server computer and uses specific values, called port numbers, to request a particular kind of service. Servers listen on ports that correspond to the services they offer. A web browser, for example, contacts a server at the destination port number 80; the web server ...