(gawkinet.info.gz) Ports

Info Catalog (gawkinet.info.gz) Basic Protocols (gawkinet.info.gz) The TCP/IP Protocols
 
 1.3.2 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
 must 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 1024 and above may be used by any application,
 although there is no promise made that a particular port number is
 always available.
 
Info Catalog (gawkinet.info.gz) Basic Protocols (gawkinet.info.gz) The TCP/IP Protocols
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