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| <h4 class="subsection">16.6.3 Internet Ports</h4> |
| |
| <p><a name="index-port-number-1732"></a> |
| A socket address in the Internet namespace consists of a machine's |
| Internet address plus a <dfn>port number</dfn> which distinguishes the |
| sockets on a given machine (for a given protocol). Port numbers range |
| from 0 to 65,535. |
| |
| <p>Port numbers less than <code>IPPORT_RESERVED</code> are reserved for standard |
| servers, such as <code>finger</code> and <code>telnet</code>. There is a database |
| that keeps track of these, and you can use the <code>getservbyname</code> |
| function to map a service name onto a port number; see <a href="Services-Database.html#Services-Database">Services Database</a>. |
| |
| <p>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 <code>IPPORT_USERRESERVED</code>; 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. |
| |
| <p>When you use a socket without specifying its address, the system |
| generates a port number for it. This number is between |
| <code>IPPORT_RESERVED</code> and <code>IPPORT_USERRESERVED</code>. |
| |
| <p>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; <code>bind</code> normally insists on |
| distinct port numbers. To reuse a port number, you must set the |
| socket option <code>SO_REUSEADDR</code>. See <a href="Socket_002dLevel-Options.html#Socket_002dLevel-Options">Socket-Level Options</a>. |
| |
| <p><a name="index-netinet_002fin_002eh-1733"></a>These macros are defined in the header file <samp><span class="file">netinet/in.h</span></samp>. |
| |
| <!-- netinet/in.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Macro: int <b>IPPORT_RESERVED</b><var><a name="index-IPPORT_005fRESERVED-1734"></a></var><br> |
| <blockquote><p>Port numbers less than <code>IPPORT_RESERVED</code> are reserved for |
| superuser use. |
| </p></blockquote></div> |
| |
| <!-- netinet/in.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Macro: int <b>IPPORT_USERRESERVED</b><var><a name="index-IPPORT_005fUSERRESERVED-1735"></a></var><br> |
| <blockquote><p>Port numbers greater than or equal to <code>IPPORT_USERRESERVED</code> are |
| reserved for explicit use; they will never be allocated automatically. |
| </p></blockquote></div> |
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