Fundamentals of Communications and Networking, 3rd Edition

Application Layer Ports and Sockets

The Application Layer is the most visible layer because it interacts with software applications that need access to network services and resources. When an application wants to request (consume) a service, such as a webpage, it sends a network message to a specific server. However, modern servers commonly provide lots of services, so how does the server know what a remote process wants? In addition to sending a request to a server, a service requester (i.e., a client) also provides a port number with the request. A port is just a number that tells the server what type of service the client is requesting. On the server, many programs run as services, which are programs that monitor a specific port for requests. ...