Service Ports

In addition to the IP address of the server, an application also needs to know how to address the specific program that it wishes to communicate with. This is accomplished by specifying a service port, a number between 1 and 65535 that uniquely identifies an application running on the system. A port can be referred to by its number, or by a name that is associated with that number. Like hostnames, service names are usually matched to port numbers through a local file, commonly called services. This file lists the logical service name, followed by the port number and protocol used by the server.

A number of standard service names are used by Internet-based applications and these are referred to as Well Known Services. These services are defined by a standards document and include common application protocols used for transferring files, accessing documents on a webserver or sending and receiving email messages. In most cases, when connecting to a service using the SocketTools libraries, they will default to the appropriate port number for that server. For example, the File Transfer Protocol control has default port values for standard and secure connections. Specifying a different port number is only necessary if you know that the server has been configured to use a non-standard port number.

It is important to remember that a service name or port number is a way to address an application running on a server. Because a particular service name is used, it doesn't guarantee that the service is available, just as dialing a telephone number doesn't guarantee that there is someone at home to answer the call.