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Ubuntu Unleashed 2014 Edition: Covering 13.10 and 14.04,Ninth Edition by Paul Hudson, Andrew Hudson, Matthew Helmke

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Ports

Most servers on your network have to perform 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 ...

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