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| 1.4 Service Ports |
| In addition to the IP address of the remote system, 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 16-bit number that uniquely identifies an application running
on the system. Instead of numbers, however, service names are usually used
instead. 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. Some common services are: |
| Service Name | Function |
|---|---|
| echo | Used to echo data back to the program that sent it. This is commonly used to test an application to make sure that a network connection can be established successfully. |
| ftp | Used to transfer files between computer systems using the File Transfer Protocol. |
| telnet | Used to provide terminal emulation services for the remote host. |
| smtp | Used to send electronic mail to a remote host using the Simple Mail Transfer Protocol. |
| Remember that a service name or port number is a way to address an application running on a remote host. 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. |
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