(libc.info.gz) Ports

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 16.6.3 Internet Ports
 ---------------------
 
 A socket address in the Internet namespace consists of a machine's
 Internet address plus a "port number" which distinguishes the sockets
 on a given machine (for a given protocol).  Port numbers range from 0
 to 65,535.
 
    Port numbers less than `IPPORT_RESERVED' are reserved for standard
 servers, such as `finger' and `telnet'.  There is a database that keeps
 track of these, and you can use the `getservbyname' function to map a
 service name onto a port number; see  Services Database.
 
    If you write a server that is not one of the standard ones defined in
 the database, you must choose a port number for it.  Use a number
 greater than `IPPORT_USERRESERVED'; such numbers are reserved for
 servers and won't ever be generated automatically by the system.
 Avoiding conflicts with servers being run by other users is up to you.
 
    When you use a socket without specifying its address, the system
 generates a port number for it.  This number is between
 `IPPORT_RESERVED' and `IPPORT_USERRESERVED'.
 
    On the Internet, it is actually legitimate to have two different
 sockets with the same port number, as long as they never both try to
 communicate with the same socket address (host address plus port
 number).  You shouldn't duplicate a port number except in special
 circumstances where a higher-level protocol requires it.  Normally, the
 system won't let you do it; `bind' normally insists on distinct port
 numbers.  To reuse a port number, you must set the socket option
 `SO_REUSEADDR'.   Socket-Level Options.
 
    These macros are defined in the header file `netinet/in.h'.
 
  -- Macro: int IPPORT_RESERVED
      Port numbers less than `IPPORT_RESERVED' are reserved for
      superuser use.
 
  -- Macro: int IPPORT_USERRESERVED
      Port numbers greater than or equal to `IPPORT_USERRESERVED' are
      reserved for explicit use; they will never be allocated
      automatically.
 

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(libc.info.gz) Protocols Database

(libc.info.gz) Internet Namespace

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