gawkinet: Ports
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1 1.3.2 TCP and UDP Ports
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1 In the postal system, the address on an envelope indicates a physical
1 location, such as a residence or office building. But there may be more
1 than one person at the location; thus you have to further quantify the
1 recipient by putting a person or company name on the envelope.
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1 In the phone system, one phone number may represent an entire
1 company, in which case you need a person's extension number in order to
1 reach that individual directly. Or, when you call a home, you have to
1 say, "May I please speak to ..." before talking to the person directly.
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1 IP networking provides the concept of addressing. An IP address
1 represents a particular computer, but no more. In order to reach the
1 mail service on a system, or the FTP or WWW service on a system, you
1 must have some way to further specify which service you want. In the
1 Internet Protocol suite, this is done with "port numbers", which
1 represent the services, much like an extension number used with a phone
1 number.
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1 Port numbers are 16-bit integers. Unix and Unix-like systems reserve
1 ports below 1024 for "well known" services, such as SMTP, FTP, and HTTP.
1 Numbers 1024 and above may be used by any application, although there is
1 no promise made that a particular port number is always available.
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