Free Windows Help Tutorials sponsored by inkjet printer cartridges dot org /title> <META HTTP-EQUIV="imagetoolbar" CONTENT="no"> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; CHARSET=ISO 8859-1"> <META name="description" Content="Hundreds of downloadable Windows help tutorials, articles and how-to guides sponsored by inkjet printer cartridges dot org."><style type="text/css">  </style> </head> <body bgcolor=#999999 vlink=blue link=blue> <span name = "INTRA" class="pg" style="top:0px; left:5px; width:714px; height:1010px"></span> <span name = "INTRA" style='top:0px; left:5px; height:1010px; width:714px' class='img'> <img src="pg0001im.png" height = "1010 width = "714" border="0"> </span> <span class='nav' style='left: 10px; top: 10px'><pre></pre></span><span name = "INTRA" style='top:51px; left:559px' class="ft5"><pre><font face='Helvetica'>Problem Solving:Internet</font></pre></span> <span name = "INTRA" style='top:107px; left:126px' class="ft3"><pre><font face='Arial'><b>Understanding</b> <b>NAT</b></font></pre></span> <span name = "INTRA" style='top:213px; left:111px' class="ft2"><pre><font face='Arial'><i><b>As</b></i> <i><b>interest</b></i> <i><b>in</b></i> <i><b>using</b></i> <i><b>the</b></i> <i><b>Internet</b></i> <i><b>grows</b></i> <i><b>and</b></i> <i><b>grows,</b></i> <i><b>Internet</b></i> <i><b>address</b></i> <i><b>space</b></i> <i><b>comes</b></i> <i><b>under</b></i></font></pre></span> <span name = "INTRA" style='top:234px; left:86px' class="ft2"><pre><font face='Arial'><i><b>greater</b></i> <i><b>demand,</b></i> <i><b>and</b></i> <i><b>the</b></i> <i><b>threat</b></i> <i><b>looms</b></i> <i><b>that</b></i> <i><b>it</b></i> <i><b>could</b></i> <i><b>run</b></i> <i><b>out.</b></i> <i><b>Network</b></i> <i><b>Address</b></i> <i><b>Translation</b></i> <i><b>is</b></i> <i><b>one</b></i></font></pre></span> <span name = "INTRA" style='top:255px; left:110px' class="ft2"><pre><font face='Arial'><i><b>method</b></i> <i><b>of</b></i> <i><b>conserving</b></i> <i><b>Internet</b></i> <i><b>address</b></i> <i><b>space</b></i> <i><b>which</b></i> <i><b>you</b></i> <i><b>can</b></i> <i><b>put</b></i> <i><b>to</b></i> <i><b>use</b></i> <i><b>on</b></i> <i><b>your</b></i> <i><b>network.</b></i></font></pre></span> <span name = "INTRA" style='top:287px; left:576px' class="ft1"><pre><font face='Arial'>By Neil Briscoe</font></pre></span> <span name = "INTRA" style='top:370px; left:77px' class="ft0"><pre><font face='Arial'><b>N</b></font></pre></span> <span name = "INTRA" style='top:375px; left:117px' class="ft1"><pre><font face='Arial'>etwork Address Translation is</font></pre></span> <span name = "INTRA" style='top:375px; left:286px' class="ft1"><pre><font face='Arial'>On the other hand, if you have ad </font></pre></span> <span name = "INTRA" style='top:375px; left:466px' class="ft2"><pre><font face='Arial'><i><b>A</b></i> <i><b>PC/Server</b></i> <i><b>Connection</b></i></font></pre></span> <span name = "INTRA" style='top:387px; left:117px' class="ft1"><pre><font face='Arial'>commonly known as NAT. In</font></pre></span> <span name = "INTRA" style='top:387px; left:272px' class="ft1"><pre><font face='Arial'>hered to RFC 1918 you will only need</font></pre></span> <span name = "INTRA" style='top:399px; left:117px' class="ft1"><pre><font face='Arial'>this article I'll explain what</font></pre></span> <span name = "INTRA" style='top:399px; left:272px' class="ft1"><pre><font face='Arial'>to change the address on the external</font></pre></span> <span name = "INTRA" style='top:397px; left:480px' class="ft1"><pre><font face='Arial'>Leaving NAT aside for the moment,</font></pre></span> <span name = "INTRA" style='top:411px; left:77px' class="ft1"><pre><font face='Arial'>NAT is and give a brief technical expla </font></pre></span> <span name = "INTRA" style='top:411px; left:272px' class="ft1"><pre><font face='Arial'>side of your router, and possibly any</font></pre></span> <span name = "INTRA" style='top:409px; left:466px' class="ft1"><pre><font face='Arial'>let's consider a connection from your</font></pre></span> <span name = "INTRA" style='top:423px; left:77px' class="ft1"><pre><font face='Arial'>nation of how it works.</font></pre></span> <span name = "INTRA" style='top:423px; left:272px' class="ft1"><pre><font face='Arial'>static HAT mappings you've used I'll</font></pre></span> <span name = "INTRA" style='top:421px; left:466px' class="ft1"><pre><font face='Arial'>PC to a server. What happens when</font></pre></span> <span name = "INTRA" style='top:435px; left:91px' class="ft1"><pre><font face='Arial'>Why do we need it? Well, it is one</font></pre></span> <span name = "INTRA" style='top:435px; left:272px' class="ft1"><pre><font face='Arial'>explain that later.</font></pre></span> <span name = "INTRA" style='top:434px; left:466px' class="ft1"><pre><font face='Arial'>you type telnet some.server.com is</font></pre></span> <span name = "INTRA" style='top:447px; left:77px' class="ft1"><pre><font face='Arial'>attempt at preserving the address</font></pre></span> <span name = "INTRA" style='top:446px; left:466px' class="ft1"><pre><font face='Arial'>that your operating system picks a port</font></pre></span> <span name = "INTRA" style='top:459px; left:77px' class="ft1"><pre><font face='Arial'>space available for use on the Internet.</font></pre></span> <span name = "INTRA" style='top:461px; left:272px' class="ft2"><pre><font face='Arial'><i><b>IP</b></i> <i><b>Connections</b></i></font></pre></span> <span name = "INTRA" style='top:458px; left:466px' class="ft1"><pre><font face='Arial'>above 1023 and assigns it to your ses </font></pre></span> <span name = "INTRA" style='top:471px; left:91px' class="ft1"><pre><font face='Arial'>Internet Service Providers no longer</font></pre></span> <span name = "INTRA" style='top:470px; left:466px' class="ft1"><pre><font face='Arial'>sion. Let's assume it picks port 1025 to</font></pre></span> <span name = "INTRA" style='top:483px; left:77px' class="ft1"><pre><font face='Arial'>use the A, B, and C classes of address.</font></pre></span> <span name = "INTRA" style='top:483px; left:286px' class="ft1"><pre><font face='Arial'>Before we delve further into NAT</font></pre></span> <span name = "INTRA" style='top:482px; left:466px' class="ft1"><pre><font face='Arial'>use.</font></pre></span> <span name = "INTRA" style='top:495px; left:77px' class="ft1"><pre><font face='Arial'>They allocate addresses in a classless</font></pre></span> <span name = "INTRA" style='top:495px; left:272px' class="ft1"><pre><font face='Arial'>we need to talk about how IP connec </font></pre></span> <span name = "INTRA" style='top:494px; left:480px' class="ft1"><pre><font face='Arial'>It connects to some.server.com on</font></pre></span> <span name = "INTRA" style='top:507px; left:77px' class="ft1"><pre><font face='Arial'>manner. What that means is that, from</font></pre></span> <span name = "INTRA" style='top:507px; left:272px' class="ft1"><pre><font face='Arial'>tions normally work.</font></pre></span> <span name = "INTRA" style='top:506px; left:466px' class="ft1"><pre><font face='Arial'>port 23 because port 23 is the port</font></pre></span> <span name = "INTRA" style='top:519px; left:77px' class="ft1"><pre><font face='Arial'>their blocks of allocated addresses,</font></pre></span> <span name = "INTRA" style='top:519px; left:290px' class="ft1"><pre><font face='Arial'>There are 65,535 ports you can</font></pre></span> <span name = "INTRA" style='top:518px; left:466px' class="ft1"><pre><font face='Arial'>reserved by IANA for telnet sessions.</font></pre></span> <span name = "INTRA" style='top:531px; left:77px' class="ft1"><pre><font face='Arial'>they will allocate you just as many ad </font></pre></span> <span name = "INTRA" style='top:531px; left:272px' class="ft1"><pre><font face='Arial'>think of them as channels for IP to</font></pre></span> <span name = "INTRA" style='top:530px; left:466px' class="ft1"><pre><font face='Arial'>But the source information in the pack </font></pre></span> <span name = "INTRA" style='top:543px; left:77px' class="ft1"><pre><font face='Arial'>dresses as you need, and no more. This</font></pre></span> <span name = "INTRA" style='top:543px; left:272px' class="ft1"><pre><font face='Arial'>choose from. (The number of ports</font></pre></span> <span name = "INTRA" style='top:542px; left:466px' class="ft1"><pre><font face='Arial'>ets that reach the server tell it that your</font></pre></span> <span name = "INTRA" style='top:555px; left:77px' class="ft1"><pre><font face='Arial'>method is known as CIDR (pro </font></pre></span> <span name = "INTRA" style='top:555px; left:272px' class="ft1"><pre><font face='Arial'>available is determined by the maxi </font></pre></span> <span name = "INTRA" style='top:554px; left:466px' class="ft1"><pre><font face='Arial'>machine is using port 1025 to service</font></pre></span> <span name = "INTRA" style='top:567px; left:77px' class="ft1"><pre><font face='Arial'>nounced Cider) which stands for</font></pre></span> <span name = "INTRA" style='top:567px; left:272px' class="ft1"><pre><font face='Arial'>mum value you can get from the 16 bits</font></pre></span> <span name = "INTRA" style='top:566px; left:466px' class="ft1"><pre><font face='Arial'>its replies.</font></pre></span> <span name = "INTRA" style='top:579px; left:77px' class="ft1"><pre><font face='Arial'>Classless Internet Domain Routing.</font></pre></span> <span name = "INTRA" style='top:579px; left:272px' class="ft1"><pre><font face='Arial'>allocated to the port number in the IP</font></pre></span> <span name = "INTRA" style='top:578px; left:480px' class="ft1"><pre><font face='Arial'>When the packets are returned from</font></pre></span> <span name = "INTRA" style='top:591px; left:77px' class="ft1"><pre><font face='Arial'>CIDR is, however, outside the scope of</font></pre></span> <span name = "INTRA" style='top:591px; left:272px' class="ft1"><pre><font face='Arial'>packet.)</font></pre></span> <span name = "INTRA" style='top:590px; left:466px' class="ft1"><pre><font face='Arial'>the server, therefore, they are aimed at</font></pre></span> <span name = "INTRA" style='top:603px; left:77px' class="ft1"><pre><font face='Arial'>this article. [See this month's Professional</font></pre></span> <span name = "INTRA" style='top:603px; left:286px' class="ft1"><pre><font face='Arial'>The first 1023 of them are reserved.</font></pre></span> <span name = "INTRA" style='top:602px; left:466px' class="ft1"><pre><font face='Arial'>the port number that was indicated in</font></pre></span> <span name = "INTRA" style='top:615px; left:77px' class="ft1"><pre><font face='Arial'>Update for an item on CIDR Ed.]</font></pre></span> <span name = "INTRA" style='top:615px; left:272px' class="ft1"><pre><font face='Arial'>They're deemed to be privileged ports</font></pre></span> <span name = "INTRA" style='top:614px; left:466px' class="ft1"><pre><font face='Arial'>the original packets in our example,</font></pre></span> <span name = "INTRA" style='top:627px; left:91px' class="ft1"><pre><font face='Arial'>All you need to know about CIDR</font></pre></span> <span name = "INTRA" style='top:627px; left:272px' class="ft1"><pre><font face='Arial'>and are reserved for what are known</font></pre></span> <span name = "INTRA" style='top:626px; left:466px' class="ft1"><pre><font face='Arial'>port 1025.</font></pre></span> <span name = "INTRA" style='top:639px; left:77px' class="ft1"><pre><font face='Arial'>for the purposes of this article is that it</font></pre></span> <span name = "INTRA" style='top:639px; left:272px' class="ft1"><pre><font face='Arial'>as the Well Known Services (WKS)</font></pre></span> <span name = "INTRA" style='top:638px; left:480px' class="ft1"><pre><font face='Arial'>If you've ever wondered how it's</font></pre></span> <span name = "INTRA" style='top:651px; left:77px' class="ft1"><pre><font face='Arial'>means that, if you change ISPs, you're</font></pre></span> <span name = "INTRA" style='top:651px; left:272px' class="ft1"><pre><font face='Arial'>such as telnet, ftp, gopher, www and</font></pre></span> <span name = "INTRA" style='top:650px; left:466px' class="ft1"><pre><font face='Arial'>possible to talk to two separate servers</font></pre></span> <span name = "INTRA" style='top:663px; left:77px' class="ft1"><pre><font face='Arial'>going to have to change all of your IP</font></pre></span> <span name = "INTRA" style='top:663px; left:272px' class="ft1"><pre><font face='Arial'>so on. So they're normally reserved for</font></pre></span> <span name = "INTRA" style='top:662px; left:466px' class="ft1"><pre><font face='Arial'>at once especially if you use a dial up</font></pre></span> <span name = "INTRA" style='top:675px; left:77px' class="ft1"><pre><font face='Arial'>addresses, unless you adhere to RFC</font></pre></span> <span name = "INTRA" style='top:675px; left:272px' class="ft1"><pre><font face='Arial'>server processes for those types of pro </font></pre></span> <span name = "INTRA" style='top:674px; left:466px' class="ft1"><pre><font face='Arial'>connection to the Internet, which</font></pre></span> <span name = "INTRA" style='top:687px; left:77px' class="ft1"><pre><font face='Arial'>1918 in the first place. Believe you me,</font></pre></span> <span name = "INTRA" style='top:687px; left:272px' class="ft1"><pre><font face='Arial'>tocol, and can't be used by client proc </font></pre></span> <span name = "INTRA" style='top:686px; left:466px' class="ft1"><pre><font face='Arial'>makes the whole procedure appear to</font></pre></span> <span name = "INTRA" style='top:699px; left:77px' class="ft1"><pre><font face='Arial'>if you have even a medium sized net </font></pre></span> <span name = "INTRA" style='top:699px; left:272px' class="ft1"><pre><font face='Arial'>esses.</font></pre></span> <span name = "INTRA" style='top:698px; left:466px' class="ft1"><pre><font face='Arial'>take place by magic it is simply be </font></pre></span> <span name = "INTRA" style='top:711px; left:77px' class="ft1"><pre><font face='Arial'>work, changing all the IP addresses on</font></pre></span> <span name = "INTRA" style='top:711px; left:286px' class="ft1"><pre><font face='Arial'>The remaining ports can be used for</font></pre></span> <span name = "INTRA" style='top:710px; left:466px' class="ft1"><pre><font face='Arial'>cause your operating system assigns,</font></pre></span> <span name = "INTRA" style='top:723px; left:77px' class="ft1"><pre><font face='Arial'>all of your machines requires a consid </font></pre></span> <span name = "INTRA" style='top:723px; left:272px' class="ft1"><pre><font face='Arial'>anything. Some of these are anyway</font></pre></span> <span name = "INTRA" style='top:722px; left:466px' class="ft1"><pre><font face='Arial'>say, port 1025 to the first session, and</font></pre></span> <span name = "INTRA" style='top:735px; left:77px' class="ft1"><pre><font face='Arial'>erable investment in time and money,</font></pre></span> <span name = "INTRA" style='top:735px; left:272px' class="ft1"><pre><font face='Arial'>used by various server processes and</font></pre></span> <span name = "INTRA" style='top:734px; left:466px' class="ft1"><pre><font face='Arial'>port 1026, say, to the second one. Each</font></pre></span> <span name = "INTRA" style='top:747px; left:77px' class="ft1"><pre><font face='Arial'>not to mention the fact that you will</font></pre></span> <span name = "INTRA" style='top:747px; left:272px' class="ft1"><pre><font face='Arial'>some are becoming Well Known </font></pre></span> <span name = "INTRA" style='top:746px; left:466px' class="ft1"><pre><font face='Arial'>server sends its responses back to your</font></pre></span> <span name = "INTRA" style='top:759px; left:77px' class="ft1"><pre><font face='Arial'>have to reboot many machines, which</font></pre></span> <span name = "INTRA" style='top:759px; left:272px' class="ft1"><pre><font face='Arial'>but until IANA (see box) grants them</font></pre></span> <span name = "INTRA" style='top:758px; left:466px' class="ft1"><pre><font face='Arial'>machine, but each using a separate</font></pre></span> <span name = "INTRA" style='top:771px; left:77px' class="ft1"><pre><font face='Arial'>can mean things fail that might other </font></pre></span> <span name = "INTRA" style='top:771px; left:272px' class="ft1"><pre><font face='Arial'>a number that use is merely an unoffi </font></pre></span> <span name = "INTRA" style='top:770px; left:466px' class="ft1"><pre><font face='Arial'>port. So, when received, your com </font></pre></span> <span name = "INTRA" style='top:783px; left:77px' class="ft1"><pre><font face='Arial'>wise have held on a little longer.</font></pre></span> <span name = "INTRA" style='top:783px; left:272px' class="ft1"><pre><font face='Arial'>cial convention.</font></pre></span> <span name = "INTRA" style='top:782px; left:466px' class="ft1"><pre><font face='Arial'>puter knows which session they were</font></pre></span> <span name = "INTRA" style='top:794px; left:466px' class="ft1"><pre><font face='Arial'>destined for and doesn't mix the ses </font></pre></span> <span name = "INTRA" style='top:806px; left:466px' class="ft1"><pre><font face='Arial'>sions up.</font></pre></span> <span name = "INTRA" style='top:817px; left:246px' class="ft2"><pre><font face='Arial'><i><b>IANA</b></i></font></pre></span> <span name = "INTRA" style='top:833px; left:102px' class="ft1"><pre><font face='Arial'>The Internet Assigned Numbers Authority (www.iana.org) is responsi </font></pre></span> <span name = "INTRA" style='top:832px; left:466px' class="ft2"><pre><font face='Arial'><i><b>The</b></i> <i><b>Need</b></i> <i><b>For</b></i> <i><b>NAT</b></i></font></pre></span> <span name = "INTRA" style='top:845px; left:88px' class="ft1"><pre><font face='Arial'>ble for allocating port numbers to Well Known Services, allocating blocks of</font></pre></span> <span name = "INTRA" style='top:854px; left:480px' class="ft1"><pre><font face='Arial'>The reason why NAT is so impor </font></pre></span> <span name = "INTRA" style='top:857px; left:88px' class="ft1"><pre><font face='Arial'>addresses to Internet Service providers (which they do via delegation), and</font></pre></span> <span name = "INTRA" style='top:866px; left:466px' class="ft1"><pre><font face='Arial'>tant is that address space under IPv4</font></pre></span> <span name = "INTRA" style='top:869px; left:88px' class="ft1"><pre><font face='Arial'>assigning Autonomous System Numbers amongst other things. Even in an</font></pre></span> <span name = "INTRA" style='top:878px; left:466px' class="ft1"><pre><font face='Arial'>(in the current version) is limited.</font></pre></span> <span name = "INTRA" style='top:881px; left:88px' class="ft1"><pre><font face='Arial'>almost unregulated environment like the Internet, responsibility for certain</font></pre></span> <span name = "INTRA" style='top:890px; left:480px' class="ft1"><pre><font face='Arial'>There are four octets, totalling 32</font></pre></span> <span name = "INTRA" style='top:893px; left:88px' class="ft1"><pre><font face='Arial'>issues has to be centralised otherwise it would all fall into chaos.</font></pre></span> <span name = "INTRA" style='top:902px; left:466px' class="ft1"><pre><font face='Arial'>bits of address space. We've tried class </font></pre></span> <span name = "INTRA" style='top:946px; left:52px' class="ft1"><pre><font face='Arial'>Issue 111 (October 1999) page 21</font></pre></span> <span name = "INTRA" style='top:946px; left:617px' class="ft1"><pre><font face='Arial'>File: P1827.1</font></pre></span> <span name = "INTRA" style='top:944px; left:261px' class="ft4"><pre><font face='Arial'><b>PC</b> <b>Network</b> <b>Advisor</b></font></pre></span> <span name = "INTRA" class='nav' style='left: 357px; top: 1010px'><pre>Next page <a href='page0002.htm'>></a></pre></span> <span name = "INTRA" style='top:1050px; left:50px' class="ft4"><pre><b><font color="#FF0000" size="3" face="Arial">New!</font><font color="#FF0000" size="3" face="Arial Narrow"> </font><font size="2" face="Arial">The best sites for quality <a href="http://www.inkjet-printer-cartridges.org">inkjet printer cartridges</a> and the best sites for <a href="http://www.cheap-inkjet-cartridges.com">cheap inkjet cartridges</a></font></b></pre></span></body> <span name = "INTRA" style='top:1100px; left:300px' class="ft4"><pre><b><font color="#FF0000" size="3" face="Arial"><a href="../index.html">Windows Help Desk Home</a> </font></b></pre></span></body></html>