Most servers on your network have more than one task. For example, web servers often have to serve both standard and secure pages. You might also be running an FTP server on the same host. For this reason, applications are provided ports to use to make “direct” connections for specific software services. These ports help TCP/IP distinguish services so that data can get to the correct application. If you check the file
/etc/services, you see the common ports and their usage. For example, for FTP, HTTP, and POP3 (email retrieval server), you see the following:
ftp 21/tcphttp 80/tcp http # WorldWideWeb HTTPpop3 110/tcp pop-3 # POP version 3
The ports defined in
/etc/services in this example are 21 for FTP, 80 for ...