Solaris™ 8 Network Administrator Certification Training Guide

INTRODUCTION

Applications running over TCP/IP networks usually act in a client-server model. Each application uses services provided by the Transport layer. The Transport layer uses a port number to distinguish among different applications. Just as IP addresses are used to distinguish among hosts on a network, port numbers are used to distinguish different network services running on the same host. Every network service uses a well-defined port number, which is used by clients to connect to that network service. For example, your email clients know that they have to connect to port 110 for POP3 service and port 25 for SMTP service. All email servers on the Internet use the same port numbers, so any client can connect to any server on the Internet. ...