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. These services are defined by a standards document and include common application protocols such as FTP, POP3, SMTP and HTTP.

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.

(Source: SocketWrench Custom Control User’s Guide and Technical Reference, Copyright 1995-1998, Catalyst Development Corporation.)

Below is a description of some well-known services

Internet Service

Port

Description

Location

Mail (send)

25

Simple Mail Transfer Protocol

RFC 821

POP3 (receive)

110

Post Office Protocol 3

RFC 1460

Usenet

119

Network News Transmission Protocol

RFC 977

World Wide Web (HTML)

80

Hypertext Transfer Protocol

RFC 2068

Gopher

70

Internet Gopher Protocol

RFC 1436

Telnet

23

Telnet Protocol specification

RFC 854

Chat

194

Internet Relay Chat

RFC 1459

Finger

79

Finger User Information Protocol

RFC 1288

FTP

21

File Transfer Protocol

RFC 959

RFC (Request for Comments) can be found at http://www.rfc-editor.org/