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(gawkinet.info)Ports

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TCP and UDP Ports
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   In the postal system, the address on an envelope indicates a physical
location, such as a residence or office building.  But there may be
more than one person at the location; thus you have to further quantify
the recipient by putting a person or company name on the envelope.

   In the phone system, one phone number may represent an entire
company, in which case you need a person's extension number in order to
reach that individual directly.  Or, when you call a home, you have to
say, "May I please speak to ..." before talking to the person directly.

   IP networking provides the concept of addressing.  An IP address
represents a particular computer, but no more.  In order to reach the
mail service on a system, or the FTP or WWW service on a system, you
have to have some way to further specify which service you want.  In
the Internet Protocol suite, this is done with "port numbers", which
represent the services, much like an extension number used with a phone
number.

   Port numbers are 16-bit integers.  Unix and Unix-like systems
reserve ports below 1024 for "well known" services, such as SMTP, FTP,
and HTTP.  Numbers above 1024 may be used by any application, although
there is no promise made that a particular port number is always
available.