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<td bgcolor="#336666"><font face="Arial" size="2" color="#FFFFFF"><b>1.1</b></font></td>
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<td bgcolor="#336666" width="100%"><b>
<font face="Arial" size="2" color="#FFFFFF">Scaling Networks with NAT and
PAT</font></b></td>
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<td bgcolor="#669999" height="25"><b><font face="Arial" size="2" color="#FFFFFF">
1.1.3</font></b></td>
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<td bgcolor="#669999" height="25" width="100%"><b>
<font face="Arial" size="2" color="#FFFFFF">Major NAT and PAT features</font></b></td>
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NAT translations can be used for a variety of purposes and can be
either dynamically or statically assigned. Static NAT is designed to
allow one-to-one mapping of local and global addresses. This is
particularly useful for hosts which must have a consistent address
that is accessible from the Internet. Such hosts may be enterprise
servers or networking devices.<p>Dynamic NAT is designed to map a
private IP address to a public address. Any IP address from a pool of
public IP addresses is assigned to a network host. Overloading, or
Port Address Translation (PAT), maps multiple private IP addresses to
a single public IP address. Multiple addresses can be mapped to a
single address because each private address is tracked by a port
number.
<font face="Arial" size="2" COLOR="#000000">
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"></font><p>PAT uses
unique source port numbers on the inside global IP address to
distinguish between translations.
<font face="Arial" size="2" COLOR="#000000">
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"> </font>The port
number is encoded in 16 bits. The total number of internal addresses
that can be translated to one external address could theoretically be
as high as 65,536 per IP address. Realistically, the number of ports
that can be assigned a single IP address is around 4000. PAT will attempt to
preserve the original source port. If this source port is already
used, PAT will assign the first available port number starting from
the beginning of the appropriate port group 0-511, 512-1023, or
1024-65535. When there are no more ports available and there is more
than one external IP address configured, PAT moves to the next IP
address to try to allocate the original source port again. This
process continues until it runs out of available ports and external IP
addresses.<p>NAT offers the following benefits:<ul>
<li>Eliminates reassigning each host a new IP address when
changing to a new ISP. NAT eliminates the need to readdress all
hosts that require external access, saving time and money.</li>
<li>Conserves addresses through application port-level
multiplexing. With PAT, internal hosts can share a single public
IP address for all external communications. In this
type of configuration, very few external addresses are required to
support many internal hosts, thereby conserving IP addresses.</li>
<li>Protects network security. Because private networks do not
advertise their addresses or internal topology, they remain
reasonably secure when used in conjunction with NAT to gain
controlled external access.</li>
</ul></font></p>
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<p><font color="#808080">
<a href="javascript:void(0)" onClick="window.open('../../activities/ch01/activity_1_1_3.html','Interactivity','width=740,height=390,toolbar=0,resizable=yes,scrollbars=0,screenX=26,screenY=38,left=26,top=38');">
<img border="0" src="../../images/media_icon.gif" width="25" height="25"></a> <b>
<a onClick="window.open('../../activities/ch01/activity_1_1_3.html','Interactivity','width=740,height=390,toolbar=0,resizable=yes,scrollbars=0,screenX=26,screenY=38,left=26,top=38');" href="javascript:void(0)">Interactive Media Activity</a></b></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>Drag and Drop: Network Address Translation with Overload
(NAT)</p>
<p>When the student has completed this activity, the student
will be able to identify the IP address and port translations
that occur when using PAT.</p>
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<p><font color="#808080">
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<b>Web Links</b></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>About NAT<p>
<a target="_blank" href="http://www.homenethelp.com/web/explain/about-NAT.asp">
http://www.homenethelp.com/web/explain/ about-NAT.asp</a></font></p>
<p>
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