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="pg0002im.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:48px' class="ft5"><pre><font face='Helvetica'>Problem Solving:Internet</font></pre></span> <span name = "INTRA" style='top:203px; left:77px' class="ft0"><pre><font face='Arial'>ing the address space, we've tried us </font></pre></span> <span name = "INTRA" style='top:203px; left:272px' class="ft0"><pre><font face='Arial'> certainly more than one. Even if two</font></pre></span> <span name = "INTRA" style='top:203px; left:466px' class="ft0"><pre><font face='Arial'>with a static NAT mapping rule. You</font></pre></span> <span name = "INTRA" style='top:215px; left:77px' class="ft0"><pre><font face='Arial'>ing CIDR, but nevertheless address</font></pre></span> <span name = "INTRA" style='top:215px; left:272px' class="ft0"><pre><font face='Arial'>separate computers happen to pick</font></pre></span> <span name = "INTRA" style='top:215px; left:466px' class="ft0"><pre><font face='Arial'>tell it the internal number in use on</font></pre></span> <span name = "INTRA" style='top:227px; left:77px' class="ft0"><pre><font face='Arial'>space will eventually run out. Our</font></pre></span> <span name = "INTRA" style='top:227px; left:272px' class="ft0"><pre><font face='Arial'>port 1025 for their respective stacks to</font></pre></span> <span name = "INTRA" style='top:227px; left:466px' class="ft0"><pre><font face='Arial'>your LAN for the relevant computer,</font></pre></span> <span name = "INTRA" style='top:239px; left:77px' class="ft0"><pre><font face='Arial'> stop gap measures have, however,</font></pre></span> <span name = "INTRA" style='top:239px; left:272px' class="ft0"><pre><font face='Arial'>use to start a session (quite likely when</font></pre></span> <span name = "INTRA" style='top:239px; left:466px' class="ft0"><pre><font face='Arial'>and you tell it the public address from</font></pre></span> <span name = "INTRA" style='top:251px; left:77px' class="ft0"><pre><font face='Arial'>succeeded in preserving the address</font></pre></span> <span name = "INTRA" style='top:251px; left:272px' class="ft0"><pre><font face='Arial'>everyone fires their computers up in</font></pre></span> <span name = "INTRA" style='top:251px; left:466px' class="ft0"><pre><font face='Arial'>your ISP assigned address block that</font></pre></span> <span name = "INTRA" style='top:263px; left:77px' class="ft0"><pre><font face='Arial'>space for longer than we all thought</font></pre></span> <span name = "INTRA" style='top:263px; left:272px' class="ft0"><pre><font face='Arial'>the morning) the router just keeps a</font></pre></span> <span name = "INTRA" style='top:263px; left:466px' class="ft0"><pre><font face='Arial'>relates to it.</font></pre></span> <span name = "INTRA" style='top:275px; left:77px' class="ft0"><pre><font face='Arial'>possible.</font></pre></span> <span name = "INTRA" style='top:275px; left:272px' class="ft0"><pre><font face='Arial'>mapping for each session it sees.</font></pre></span> <span name = "INTRA" style='top:275px; left:480px' class="ft0"><pre><font face='Arial'>Now that this is properly set up, if</font></pre></span> <span name = "INTRA" style='top:287px; left:91px' class="ft0"><pre><font face='Arial'>RFC 1918 provides the rules for us </font></pre></span> <span name = "INTRA" style='top:287px; left:286px' class="ft0"><pre><font face='Arial'>The advantages of NAT are that it</font></pre></span> <span name = "INTRA" style='top:287px; left:466px' class="ft0"><pre><font face='Arial'>someone ftps to the public IP address</font></pre></span> <span name = "INTRA" style='top:299px; left:77px' class="ft0"><pre><font face='Arial'>ing a set of reserved numbers guaran </font></pre></span> <span name = "INTRA" style='top:299px; left:272px' class="ft0"><pre><font face='Arial'>works, with RFC 1918, to conserve the</font></pre></span> <span name = "INTRA" style='top:299px; left:466px' class="ft0"><pre><font face='Arial'>listed in the static NAT mapping you</font></pre></span> <span name = "INTRA" style='top:311px; left:77px' class="ft0"><pre><font face='Arial'>teed never to be in use on the Internet.</font></pre></span> <span name = "INTRA" style='top:311px; left:272px' class="ft0"><pre><font face='Arial'>address space. The disadvantages are</font></pre></span> <span name = "INTRA" style='top:311px; left:466px' class="ft0"><pre><font face='Arial'>made, your router will re write the</font></pre></span> <span name = "INTRA" style='top:323px; left:77px' class="ft0"><pre><font face='Arial'>The beauty of it is that every company</font></pre></span> <span name = "INTRA" style='top:323px; left:272px' class="ft0"><pre><font face='Arial'>that it slows down the process of trans </font></pre></span> <span name = "INTRA" style='top:323px; left:466px' class="ft0"><pre><font face='Arial'>packets and transmit them inside to the</font></pre></span> <span name = "INTRA" style='top:335px; left:77px' class="ft0"><pre><font face='Arial'>that connects can pick from the same</font></pre></span> <span name = "INTRA" style='top:335px; left:272px' class="ft0"><pre><font face='Arial'>mission, and limits the total number of</font></pre></span> <span name = "INTRA" style='top:335px; left:466px' class="ft0"><pre><font face='Arial'>correct machine on your LAN.</font></pre></span> <span name = "INTRA" style='top:347px; left:77px' class="ft0"><pre><font face='Arial'>set of reserved numbers, and it doesn't</font></pre></span> <span name = "INTRA" style='top:347px; left:272px' class="ft0"><pre><font face='Arial'>sessions to the router to slightly less</font></pre></span> <span name = "INTRA" style='top:347px; left:480px' class="ft0"><pre><font face='Arial'>To keep things relatively simple, in</font></pre></span> <span name = "INTRA" style='top:359px; left:77px' class="ft0"><pre><font face='Arial'>matter.</font></pre></span> <span name = "INTRA" style='top:359px; left:272px' class="ft0"><pre><font face='Arial'>than 65,000 at any one time. That's not</font></pre></span> <span name = "INTRA" style='top:359px; left:466px' class="ft0"><pre><font face='Arial'>describing static NAT I have spoken of</font></pre></span> <span name = "INTRA" style='top:371px; left:91px' class="ft0"><pre><font face='Arial'>However, as these numbers are not</font></pre></span> <span name = "INTRA" style='top:371px; left:272px' class="ft0"><pre><font face='Arial'>really a disadvantage, however, since</font></pre></span> <span name = "INTRA" style='top:371px; left:466px' class="ft0"><pre><font face='Arial'>you providing a computer on your net </font></pre></span> <span name = "INTRA" style='top:383px; left:77px' class="ft0"><pre><font face='Arial'>routed around the Internet at all we</font></pre></span> <span name = "INTRA" style='top:383px; left:272px' class="ft0"><pre><font face='Arial'>unless you had an absolutely vast ex </font></pre></span> <span name = "INTRA" style='top:383px; left:466px' class="ft0"><pre><font face='Arial'>work, or on your LAN, as a server you</font></pre></span> <span name = "INTRA" style='top:395px; left:77px' class="ft0"><pre><font face='Arial'>have to have a method of transmitting</font></pre></span> <span name = "INTRA" style='top:395px; left:272px' class="ft0"><pre><font face='Arial'>ternal pipe you wouldn't want to be</font></pre></span> <span name = "INTRA" style='top:395px; left:466px' class="ft0"><pre><font face='Arial'>allow unknown outsiders to connect to</font></pre></span> <span name = "INTRA" style='top:408px; left:77px' class="ft0"><pre><font face='Arial'>packets around the Internet, and re </font></pre></span> <span name = "INTRA" style='top:408px; left:272px' class="ft0"><pre><font face='Arial'>running that many sessions through</font></pre></span> <span name = "INTRA" style='top:408px; left:466px' class="ft0"><pre><font face='Arial'>via the public Internet. From a security</font></pre></span> <span name = "INTRA" style='top:420px; left:77px' class="ft0"><pre><font face='Arial'>ceiving the replies, and then sending</font></pre></span> <span name = "INTRA" style='top:420px; left:272px' class="ft0"><pre><font face='Arial'>your router anyway.</font></pre></span> <span name = "INTRA" style='top:420px; left:466px' class="ft0"><pre><font face='Arial'>perspective this is a very unwise thing</font></pre></span> <span name = "INTRA" style='top:432px; left:77px' class="ft0"><pre><font face='Arial'>the packets inwards to our network to</font></pre></span> <span name = "INTRA" style='top:432px; left:466px' class="ft0"><pre><font face='Arial'>to do. The best advice is to get a router</font></pre></span> <span name = "INTRA" style='top:444px; left:77px' class="ft0"><pre><font face='Arial'>the correct computer on our LAN, and</font></pre></span> <span name = "INTRA" style='top:445px; left:272px' class="ft1"><pre><font face='Arial'><i><b>Dynamic</b></i> <i><b>And</b></i> <i><b>Static</b></i> <i><b>NAT</b></i></font></pre></span> <span name = "INTRA" style='top:444px; left:466px' class="ft0"><pre><font face='Arial'>with two Ethernet ports, put all your</font></pre></span> <span name = "INTRA" style='top:456px; left:77px' class="ft0"><pre><font face='Arial'>the correct session on that computer.</font></pre></span> <span name = "INTRA" style='top:456px; left:466px' class="ft0"><pre><font face='Arial'>company private machines on one seg </font></pre></span> <span name = "INTRA" style='top:468px; left:77px' class="ft0"><pre><font face='Arial'>This is where NAT comes in.</font></pre></span> <span name = "INTRA" style='top:468px; left:286px' class="ft0"><pre><font face='Arial'>Finally a quick word on the differ </font></pre></span> <span name = "INTRA" style='top:468px; left:466px' class="ft0"><pre><font face='Arial'>ment, and put the machines you wish</font></pre></span> <span name = "INTRA" style='top:480px; left:91px' class="ft0"><pre><font face='Arial'>To use NAT, the router which con </font></pre></span> <span name = "INTRA" style='top:480px; left:272px' class="ft0"><pre><font face='Arial'>ence between dynamic NAT and static</font></pre></span> <span name = "INTRA" style='top:480px; left:466px' class="ft0"><pre><font face='Arial'>to be public on the other segment. Con </font></pre></span> <span name = "INTRA" style='top:492px; left:77px' class="ft0"><pre><font face='Arial'>nects your LAN to the Internet will</font></pre></span> <span name = "INTRA" style='top:492px; left:272px' class="ft0"><pre><font face='Arial'>NAT. What has been defined so far is</font></pre></span> <span name = "INTRA" style='top:492px; left:466px' class="ft0"><pre><font face='Arial'>figure the router with suitable ACLs.</font></pre></span> <span name = "INTRA" style='top:504px; left:77px' class="ft0"><pre><font face='Arial'>have two addresses. On the LAN side,</font></pre></span> <span name = "INTRA" style='top:504px; left:272px' class="ft0"><pre><font face='Arial'>known as dynamic NAT all packets</font></pre></span> <span name = "INTRA" style='top:516px; left:77px' class="ft0"><pre><font face='Arial'>it will have an address from the par </font></pre></span> <span name = "INTRA" style='top:516px; left:272px' class="ft0"><pre><font face='Arial'>leaving your LAN for the Internet con </font></pre></span> <span name = "INTRA" style='top:528px; left:77px' class="ft0"><pre><font face='Arial'>ticular RFC 1918 address range you</font></pre></span> <span name = "INTRA" style='top:528px; left:272px' class="ft0"><pre><font face='Arial'>tain the same source IP address, which</font></pre></span> <span name = "INTRA" style='top:540px; left:77px' class="ft0"><pre><font face='Arial'>chose to use and, on the Internet side,</font></pre></span> <span name = "INTRA" style='top:540px; left:272px' class="ft0"><pre><font face='Arial'>is the public one assigned to your</font></pre></span> <span name = "INTRA" style='top:552px; left:77px' class="ft0"><pre><font face='Arial'>it will have an address assigned to you</font></pre></span> <span name = "INTRA" style='top:552px; left:272px' class="ft0"><pre><font face='Arial'>router.</font></pre></span> <span name = "INTRA" style='top:564px; left:77px' class="ft0"><pre><font face='Arial'>by your current ISP.</font></pre></span> <span name = "INTRA" style='top:564px; left:286px' class="ft0"><pre><font face='Arial'>There is one drawback with this. If</font></pre></span> <span name = "INTRA" style='top:576px; left:91px' class="ft0"><pre><font face='Arial'>Now let's look at that example</font></pre></span> <span name = "INTRA" style='top:576px; left:272px' class="ft0"><pre><font face='Arial'>the router is the only device with a</font></pre></span> <span name = "INTRA" style='top:588px; left:77px' class="ft0"><pre><font face='Arial'>again. Your machine sends out a</font></pre></span> <span name = "INTRA" style='top:588px; left:272px' class="ft0"><pre><font face='Arial'>public address, then there is no way for</font></pre></span> <span name = "INTRA" style='top:600px; left:77px' class="ft0"><pre><font face='Arial'>packet aimed at some.server.com. The</font></pre></span> <span name = "INTRA" style='top:600px; left:272px' class="ft0"><pre><font face='Arial'>you to provide information services on</font></pre></span> <span name = "INTRA" style='top:612px; left:77px' class="ft0"><pre><font face='Arial'>source IP and source port are in the</font></pre></span> <span name = "INTRA" style='top:612px; left:272px' class="ft0"><pre><font face='Arial'>any computer on your network.</font></pre></span> <span name = "INTRA" style='top:624px; left:77px' class="ft0"><pre><font face='Arial'>packet, just as before, together with the</font></pre></span> <span name = "INTRA" style='top:624px; left:286px' class="ft0"><pre><font face='Arial'>Supposing that, despite being a</font></pre></span> <span name = "INTRA" style='top:636px; left:77px' class="ft0"><pre><font face='Arial'>destination port and IP address.</font></pre></span> <span name = "INTRA" style='top:636px; left:272px' class="ft0"><pre><font face='Arial'>good network citizen and using RFC</font></pre></span> <span name = "INTRA" style='top:648px; left:91px' class="ft0"><pre><font face='Arial'>When it arrives at the router, the</font></pre></span> <span name = "INTRA" style='top:648px; left:272px' class="ft0"><pre><font face='Arial'>1918 addressing, you nevertheless</font></pre></span> <span name = "INTRA" style='top:660px; left:77px' class="ft0"><pre><font face='Arial'>router will de encapsulate the packet,</font></pre></span> <span name = "INTRA" style='top:660px; left:272px' class="ft0"><pre><font face='Arial'>wish to provide a Web server, or an ftp</font></pre></span> <span name = "INTRA" style='top:655px; left:600px' class="ft3"><pre><font face='Arial'><b>PCNA</b></font></pre></span> <span name = "INTRA" style='top:672px; left:77px' class="ft0"><pre><font face='Arial'>and re write it. The packet it sends out</font></pre></span> <span name = "INTRA" style='top:672px; left:272px' class="ft0"><pre><font face='Arial'>repository, for example. You can't, be </font></pre></span> <span name = "INTRA" style='top:684px; left:77px' class="ft0"><pre><font face='Arial'>onto the Internet will contain the</font></pre></span> <span name = "INTRA" style='top:684px; left:272px' class="ft0"><pre><font face='Arial'>cause no one on the Internet has any</font></pre></span> <span name = "INTRA" style='top:696px; left:77px' class="ft0"><pre><font face='Arial'>router's public IP address, a source</font></pre></span> <span name = "INTRA" style='top:696px; left:272px' class="ft0"><pre><font face='Arial'>way of specifying that they want to</font></pre></span> <span name = "INTRA" style='top:708px; left:77px' class="ft0"><pre><font face='Arial'>port allocated from the router's list of</font></pre></span> <span name = "INTRA" style='top:708px; left:272px' class="ft0"><pre><font face='Arial'>connect to the specific computer con </font></pre></span> <span name = "INTRA" style='top:720px; left:77px' class="ft0"><pre><font face='Arial'>available ports, and the same destina </font></pre></span> <span name = "INTRA" style='top:720px; left:272px' class="ft0"><pre><font face='Arial'>taining the server.</font></pre></span> <span name = "INTRA" style='top:732px; left:77px' class="ft0"><pre><font face='Arial'>tion IP address and port number that</font></pre></span> <span name = "INTRA" style='top:732px; left:286px' class="ft0"><pre><font face='Arial'>However, static NAT allows this to</font></pre></span> <span name = "INTRA" style='top:744px; left:77px' class="ft0"><pre><font face='Arial'>your machine generated.</font></pre></span> <span name = "INTRA" style='top:744px; left:272px' class="ft0"><pre><font face='Arial'>happen. First, you will need to get your</font></pre></span> <span name = "INTRA" style='top:756px; left:91px' class="ft0"><pre><font face='Arial'>The router will also add an entry</font></pre></span> <span name = "INTRA" style='top:756px; left:272px' class="ft0"><pre><font face='Arial'>ISP to allocate you a block of public</font></pre></span> <span name = "INTRA" style='top:768px; left:77px' class="ft0"><pre><font face='Arial'>into a table it keeps, which maps the</font></pre></span> <span name = "INTRA" style='top:768px; left:272px' class="ft0"><pre><font face='Arial'>addresses. Using CIDR, most ISPs will</font></pre></span> <span name = "INTRA" style='top:780px; left:77px' class="ft0"><pre><font face='Arial'>internal address and source port</font></pre></span> <span name = "INTRA" style='top:780px; left:272px' class="ft0"><pre><font face='Arial'>allocate you a block of eight addresses.</font></pre></span> <span name = "INTRA" style='top:792px; left:77px' class="ft0"><pre><font face='Arial'>number your machine generated</font></pre></span> <span name = "INTRA" style='top:792px; left:272px' class="ft0"><pre><font face='Arial'>Because of certain rules which we</font></pre></span> <span name = "INTRA" style='top:804px; left:77px' class="ft0"><pre><font face='Arial'>against the port number it allocated to</font></pre></span> <span name = "INTRA" style='top:804px; left:272px' class="ft0"><pre><font face='Arial'>won't go into here, you will have five</font></pre></span> <span name = "INTRA" style='top:816px; left:77px' class="ft0"><pre><font face='Arial'>this session. Therefore, when the ma </font></pre></span> <span name = "INTRA" style='top:816px; left:272px' class="ft0"><pre><font face='Arial'>of these eight available for use for static</font></pre></span> <span name = "INTRA" style='top:828px; left:77px' class="ft0"><pre><font face='Arial'>chine some.server.com sends a reply</font></pre></span> <span name = "INTRA" style='top:828px; left:272px' class="ft0"><pre><font face='Arial'>mapping.</font></pre></span> <span name = "INTRA" style='top:840px; left:77px' class="ft0"><pre><font face='Arial'>packet to the router, the router can</font></pre></span> <span name = "INTRA" style='top:840px; left:286px' class="ft0"><pre><font face='Arial'>The computer on your LAN with</font></pre></span> <span name = "INTRA" style='top:843px; left:512px' class="ft2"><pre><font face='Arial'><b>The</b> <b>Author</b></font></pre></span> <span name = "INTRA" style='top:852px; left:77px' class="ft0"><pre><font face='Arial'>quickly work out how it needs to re </font></pre></span> <span name = "INTRA" style='top:852px; left:272px' class="ft0"><pre><font face='Arial'>the service you wish to make available</font></pre></span> <span name = "INTRA" style='top:864px; left:77px' class="ft0"><pre><font face='Arial'>write the packet before transmitting it</font></pre></span> <span name = "INTRA" style='top:864px; left:272px' class="ft0"><pre><font face='Arial'>still gets assigned an RFC 1918 address</font></pre></span> <span name = "INTRA" style='top:866px; left:477px' class="ft0"><pre><font face='Arial'>Neil Briscoe is a network consult </font></pre></span> <span name = "INTRA" style='top:876px; left:77px' class="ft0"><pre><font face='Arial'>back on the LAN.</font></pre></span> <span name = "INTRA" style='top:876px; left:272px' class="ft0"><pre><font face='Arial'> otherwise, you wouldn't be able to</font></pre></span> <span name = "INTRA" style='top:878px; left:477px' class="ft0"><pre><font face='Arial'>ant and can be contacted as</font></pre></span> <span name = "INTRA" style='top:888px; left:91px' class="ft0"><pre><font face='Arial'>This works well because most</font></pre></span> <span name = "INTRA" style='top:888px; left:272px' class="ft0"><pre><font face='Arial'>connect to it from your own comput </font></pre></span> <span name = "INTRA" style='top:890px; left:477px' class="ft0"><pre><font face='Arial'>neil.briscoe@itp journals.com.</font></pre></span> <span name = "INTRA" style='top:900px; left:77px' class="ft0"><pre><font face='Arial'>LANs have many computers on them</font></pre></span> <span name = "INTRA" style='top:900px; left:272px' class="ft0"><pre><font face='Arial'>ers. Next, you configure the router</font></pre></span> <span name = "INTRA" style='top:946px; left:52px' class="ft0"><pre><font face='Arial'>File: P1827.2</font></pre></span> <span name = "INTRA" style='top:946px; left:521px' class="ft0"><pre><font face='Arial'>Issue 111 (October 1999) page 22</font></pre></span> <span name = "INTRA" style='top:944px; left:260px' 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><a href='page0001.htm'><</a> Next page <a href='page0001.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>