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%>?utm_source=email&utm_medium=share&utm_campaign=hsw_share <% title %> </script> </div> <div id="editorial-main"> <div class="editorial-header"> </div> <article id="editorial-body" class="editorial-body infinite-container break-words"> <script defer src="https://rampjs-cdn.system1.com/ramp.js"></script> <script> HSW.utilities.docReady(() => { const rsocSegments = {"default":"c2showstuffworks0004","EXPO1922_ABOVE_TITLE":"c2showstuffworks0014","EXPO1922_BELOW_IMAGE":"c2showstuffworks0024"}; if (GACtest === 'EXPO1922_CONTROL' || typeof rsocSegments[GACtest] === 'undefined') { return; } (function(w,r){w[r]=w[r]||function(){(w[r]['q']=w[r]['q']||[]).push(arguments)},w[r]['t']=1*new Date()})(window,'_rampJs'); let rampSetup = { content_experiment_id: GACtest, init: { segment: rsocSegments[GACtest] }, terms: rampKeywords }; if (HSW.env != 'prod') { rampSetup.referrer = "https://howstuffworks.com/"; rampSetup.testMode = true; } _rampJs(rampSetup); }); </script> <div class="page-content"> <div class="page-body se" id="page0" data-slide="0" data-track-gtm="Content"> <div class="editorial-sidebar mx-auto mb-6 md:float-left md:ml-0 md:mr-6 md:mb-4" style="max-width:290px;" data-track-gtm="Editorial Sidebar"> <figure class="fragment-media lead-image media-vertical" style="max-width: 290px;" > <picture> <source " srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1yb3V0ZXIuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyOTB9LCJ0b0Zvcm1hdCI6ImF2aWYifX0=" type="image/avif"> <source " srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1yb3V0ZXIuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyOTB9fX0="> <img class=" "src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1yb3V0ZXIuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyOTB9fX0=" alt="Diagram of a NAT router directing traffic in between private and public networks." width="290" height="105" border="0"> </picture> <noscript> "<img src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1yb3V0ZXIuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyOTB9fX0=" width="290" alt="Diagram of a NAT router directing traffic in between private and public networks." border="0">" </noscript> <figcaption class="media-body-wrap bg-accent-3 p-4 w-max-full text-sm text-left"> Network Address Translation helps improve security by reusing IP addresses. The NAT router translates traffic coming into and leaving the private network. See more <a href="https://computer.howstuffworks.com/computer-networking-pictures.htm">pictures of computer networking</a>. </figcaption> </figure> </div> <div id="rampjs_slot1" class="primary-slot inset mb-6 rsoc-slot rsoc-after-lead"></div> <p>If you are reading this article, you are most likely connected to the internet and viewing it at the HowStuffWorks website. Therefore, it's very probable that you are using Network Address Translation (<b>NAT</b>) right now.</p> <p>The Internet has grown larger than anyone ever imagined it could be. Although the exact size is unknown, the current estimate is that there are more than <a href="https://www.statista.com/statistics/617136/digital-population-worldwide/" class="ext-link">5.35 billion</a> users actively on the internet. So what does the size of the internet have to do with NAT? Everything! In this article, you will learn how NAT can benefit you. But first, let's take a closer look at NAT and exactly what it can do...</p> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-99"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline1" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline1" class="ad-tag h-max-250"></div></div> </div> </div> <div id="rampjs_slot1" class="secondary-slot mb-6 rsoc-slot rsoc-after-lead"></div> <div class="hidden lg:block print:hidden mb-6 text-center w-full clear-both" id="inline-video-wrap" style="height: 403px;"> <div class="mx-auto w-min"> <div id="unruly-video-container" style="width: 717px; height: 403px;"></div> </div> </div> <script> (function () { if (!HSW.utilities.isMobile() && HSW.utilities.hashArgs['video-ad'] !== 'off' && userData.adsActive) { HSW.utilities.delayLoad(() => { HSW.pq.add(() => { if (userData.adsActive) { HSW.utilities.loadScript('https://tg1.unrulyvideo.com/api/adserver/spt?AV_TAGID=642d0774bf980e733c0bdd08&AV_PUBLISHERID=63a310da17f8afa0e105a8f5', true, () => {}); } else { console.log('Unruly player blocked'); } }); }); } else { let el = document.getElementById('inline-video-wrap'); el.parentNode.removeChild(el); } })(); </script> </div> </div> <div class="clear-both"> <div class="toc inline-block bg-accent-4 py-4 px-8 text-2xl text-lighter-gray mb-8 print:hidden" data-track-gtm="TOC"> <strong>Contents</strong> <ol class="list-outside list-decimal text-lg mt-2 ml-8 text-primary font-bold"> <li class="pb-2 pl-2 text-lg"> <a data-target="pt1" class="text-lighter-gray hover:text-green font-normal" href="#pt1">When the Global IP Address Pool Runs Dry</a> </li> <li class="pb-2 pl-2 text-lg"> <a data-target="pt2" class="text-lighter-gray hover:text-green font-normal" href="#pt2">What Does NAT Do?</a> </li> <li class="pb-2 pl-2 text-lg"> <a data-target="pt3" class="text-lighter-gray hover:text-green font-normal" href="#pt3">NAT Configuration</a> </li> <li class="pb-2 pl-2 text-lg"> <a data-target="pt4" class="text-lighter-gray hover:text-green font-normal" href="#pt4">The Connections of a NAT Overload</a> </li> <li class="pb-2 pl-2 text-lg"> <a data-target="pt5" class="text-lighter-gray hover:text-green font-normal" href="#pt5">Dynamic Network Address Translation</a> </li> <li class="pb-2 pl-2 text-lg"> <a data-target="pt6" class="text-lighter-gray hover:text-green font-normal" href="#pt6">How NAT Overloading Works</a> </li> <li class="pb-2 pl-2 text-lg"> <a data-target="pt7" class="text-lighter-gray hover:text-green font-normal" href="#pt7">Stub Domains</a> </li> <li class="pb-2 pl-2 text-lg"> <a data-target="pt8" class="text-lighter-gray hover:text-green font-normal" href="#pt8">How IP Address Information is Classified</a> </li> <li class="pb-2 pl-2 text-lg"> <a data-target="pt9" class="text-lighter-gray hover:text-green font-normal" href="#pt9">Security and Administration</a> </li> <li class="pb-2 pl-2 text-lg"> <a data-target="pt10" class="text-lighter-gray hover:text-green font-normal" href="#pt10">A Note on Proxy Servers</a> </li> <li class="pb-2 pl-2 text-lg"> <a data-target="pt11" class="text-lighter-gray hover:text-green font-normal" href="#pt11">Multi-homing</a> </li> </ol> </div> </div> <div class="page-content relative clear-both" id="page-wrap1" x-data="{ pageVisible : true }"><a class="absolute left-0 top-n56" name="pt1"></a> <h2 class="page-title firePageview text-2xl mb-6 font-bold se" data-page-nbr="1" data-logged="false" data-page-url="nat1.htm"> <span class="flex cursor-pointer " @click="pageVisible = !pageVisible" aria-expanded="true" aria-controls="page1" role="button"> <svg class="mt-3 mr-3 print:hidden shrink-0 fill-current stroke-current transition ease-in-out duration-200 transform" :class="{ 'rotate-180' : !pageVisible, '' : pageVisible }" xmlns='http://www.w3.org/2000/svg' width='22' height='10' viewbox='0 0 28.396 13.211'><path d='M4398.8,5158.252l13.224,10.507,12.357-10.507' transform='translate(-4397.399 -5156.842)' fill='none' stroke-linecap='round' stroke-width='2'/></svg> <span>When the Global IP Address Pool Runs Dry</span> </span> </h2> <div class="page-body se" id="page1" data-slide="1" data-track-gtm="Content" x-show.transition="pageVisible === true"> <p>For a computer to communicate with other computers and <a href="https://computer.howstuffworks.com/web-server.htm">web servers</a> on the internet, it must have an IP address. An IP address (IP stands for Internet Protocol) is a unique 32-bit number that identifies the location of your computer on a network. Basically, it works like your street address — as a way to find out exactly where you are and deliver information to you.</p> <p>When IP addressing first came out, everyone thought that there were plenty of addresses to cover any need. Theoretically, you could have <a href="https://computer.howstuffworks.com/internet/basics/what-is-an-ip-address.htm">4,294,967,296 unique addresses</a> (2<sup>32</sup>). The actual number of public IP addresses is smaller (somewhere between 3.2 and 3.3 billion) because of the way that the addresses are separated into classes, and because some addresses are set aside for multicasting, testing, or other special uses.</p> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-99"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline3" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline3" class="ad-tag h-max-250"></div></div> </div> </div> <p>With the explosion of the Internet and the increase in <a href="https://computer.howstuffworks.com/home-network.htm">home networks</a> and business networks, the number of available IP addresses is simply not enough. The obvious solution is to redesign the public IP address format to allow for more possible addresses. This is being developed (called IPv6), but will take several years to implement because it requires modification of the entire infrastructure of the internet.</p> <p> This is where NAT (<a href="http://www.faqs.org/rfcs/rfc1631.html" class="ext-link">RFC 1631</a>) comes to the rescue. Network Address Translation allows a single device, such as a <a href="https://computer.howstuffworks.com/router.htm">router</a>, to act as an agent between the Internet (or "public network") and a local (or "private") network. This means that only a single, unique IP address is required to represent an entire group of computers. But the shortage of public IP addresses is only one reason to use NAT.</p> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-14"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline4" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline4" class="ad-tag "></div></div> </div> </div> </div> </div> <div class="page-content relative clear-both" id="page-wrap2" x-data="{ pageVisible : true }"><a class="absolute left-0 top-n56" name="pt2"></a> <h2 class="page-title firePageview text-2xl mb-6 font-bold se" data-page-nbr="2" data-logged="false" data-page-url="nat2.htm"> <span class="flex cursor-pointer " @click="pageVisible = !pageVisible" aria-expanded="true" aria-controls="page2" role="button"> <svg class="mt-3 mr-3 print:hidden shrink-0 fill-current stroke-current transition ease-in-out duration-200 transform" :class="{ 'rotate-180' : !pageVisible, '' : pageVisible }" xmlns='http://www.w3.org/2000/svg' width='22' height='10' viewbox='0 0 28.396 13.211'><path d='M4398.8,5158.252l13.224,10.507,12.357-10.507' transform='translate(-4397.399 -5156.842)' fill='none' stroke-linecap='round' stroke-width='2'/></svg> <span>What Does NAT Do?</span> </span> </h2> <div class="page-body se" id="page2" data-slide="2" data-track-gtm="Content" x-show.transition="pageVisible === true"> <p>NAT is like the receptionist in a large office. Let's say you have left instructions with the receptionist not to forward any calls to you unless you request it. Later on, you call a potential client and leave a message for that client to call you back. You tell the receptionist that you are expecting a call from this client and to put her through.</p> <p>The client calls the main number to your office, which is the only number the client knows. When the client tells the receptionist that she is looking for you, the receptionist checks a lookup table that matches your name with your extension. The receptionist knows that you requested this call, and therefore forwards the caller to your extension.</p> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-99"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline5" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline5" class="ad-tag h-max-250"></div></div> </div> </div> <p>Developed by Cisco, Network Address Translation is used by a device (<a href="https://computer.howstuffworks.com/firewall.htm">firewall</a>, router, or computer that sits between an internal network and the rest of the world). NAT has many forms and can work in several ways:</p> <figure class="fragment-media clear-both w-max-full mx-auto mb-6 md:float-left md:ml-0 md:mr-6 md:mb-4" style="max-width: 285px;" style="max-width: 285px;"> <picture class="lazyload"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1zdGF0aWMuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyODV9LCJ0b0Zvcm1hdCI6ImF2aWYifX0=" type="image/avif"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1zdGF0aWMuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyODV9fX0="> <img class=" bg-gray-light bg-opacity-50" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAJCAQAAACRI2S5AAAAEElEQVR42mNkIAAYRxWAAQAG9gAKqv6+AwAAAABJRU5ErkJggg==" data-src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1zdGF0aWMuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyODV9fX0=" alt="Diagram showing how IP addresses translate in static NAT." width="285" height="92" border="0"> </picture> <noscript> "<img src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1zdGF0aWMuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyODV9fX0=" width="285" alt="Diagram showing how IP addresses translate in static NAT." border="0">" </noscript> <figcaption class="media-body-wrap bg-accent-3 p-4 w-max-full text-sm text-left"> In static NAT, the computer with the IP address of 192.168.32.10 will always translate to 213.18.123.110. </figcaption> </figure> <div class="list"> <ul><li><span><b>Static NAT</b> - Mapping an unregistered IP address to a registered IP address on a one-to-one basis. Particularly useful when a device needs to be accessible from outside the network.</span></li></ul> </div> <figure class="fragment-media clear-both w-max-full mx-auto mb-6 md:float-left md:ml-0 md:mr-6 md:mb-4" style="max-width: 285px;" style="max-width: 285px;"> <picture class="lazyload"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1keW5hbWljLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6Mjg1fSwidG9Gb3JtYXQiOiJhdmlmIn19" type="image/avif"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1keW5hbWljLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6Mjg1fX19"> <img class=" bg-gray-light bg-opacity-50" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAJCAQAAACRI2S5AAAAEElEQVR42mNkIAAYRxWAAQAG9gAKqv6+AwAAAABJRU5ErkJggg==" data-src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1keW5hbWljLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6Mjg1fX19" alt="Diagram showing how IP addresses translate in static NAT." width="285" height="127" border="0"> </picture> <noscript> "<img src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1keW5hbWljLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6Mjg1fX19" width="285" alt="Diagram showing how IP addresses translate in static NAT." border="0">" </noscript> <figcaption class="media-body-wrap bg-accent-3 p-4 w-max-full text-sm text-left"> In dynamic NAT, the computer with the IP address 192.168.32.10 will translate to the first available address in the range from 213.18.123.100 to 213.18.123.150. </figcaption> </figure> <div class="list"> <ul><li><span><b>Dynamic NAT</b> - Maps an unregistered IP address to a registered IP address from a group of registered IP addresses.</span></li></ul> </div> <div class="list"> <ul><li><span><b>Overloading</b> - A form of dynamic NAT that maps multiple unregistered IP addresses to a single registered IP address by using different ports. This is known also as PAT (Port Address Translation), single IP address NAT or port-level multiplexed NAT.</span></li></ul> </div> <figure class="fragment-media clear-both w-max-full mx-auto mb-6 md:float-left md:ml-0 md:mr-6 md:mb-4" style="max-width: 285px;" style="max-width: 285px;"> <picture class="lazyload"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1vdmVybG9hZC5qcGciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjI4NX0sInRvRm9ybWF0IjoiYXZpZiJ9fQ==" type="image/avif"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1vdmVybG9hZC5qcGciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjI4NX19fQ=="> <img class=" bg-gray-light bg-opacity-50" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAJCAQAAACRI2S5AAAAEElEQVR42mNkIAAYRxWAAQAG9gAKqv6+AwAAAABJRU5ErkJggg==" data-src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1vdmVybG9hZC5qcGciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjI4NX19fQ==" alt="Diagram showing how each computer is translated to the same IP address in NAT overloading." width="285" height="92" border="0"> </picture> <noscript> "<img src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1vdmVybG9hZC5qcGciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjI4NX19fQ==" width="285" alt="Diagram showing how each computer is translated to the same IP address in NAT overloading." border="0">" </noscript> <figcaption class="media-body-wrap bg-accent-3 p-4 w-max-full text-sm text-left"> In overloading, each computer on the private network is translated to the same IP address (213.18.123.100), but with a different port number assignment. </figcaption> </figure> <div class="list"> <ul><li><span><b>Overlapping</b> - When the IP addresses used on your internal network are registered IP addresses in use on another network, the router must maintain a lookup table of these addresses so that it can intercept them and replace them with registered unique IP addresses. It is important to note that the NAT router must translate the "internal" addresses to registered unique addresses as well as translate the "external" registered addresses to addresses that are unique to the private network. This can be done either through static NAT or by using <a href="https://computer.howstuffworks.com/dns.htm">DNS</a> and implementing dynamic NAT.</span></li></ul> </div> <figure class="fragment-media clear-both w-max-full mx-auto mb-6 md:float-left md:ml-0 md:mr-6 md:mb-4" style="max-width: 285px;" style="max-width: 285px;"> <picture class="lazyload"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1vdmVybGFwLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6Mjg1fSwidG9Gb3JtYXQiOiJhdmlmIn19" type="image/avif"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1vdmVybGFwLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6Mjg1fX19"> <img class=" bg-gray-light bg-opacity-50" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAJCAQAAACRI2S5AAAAEElEQVR42mNkIAAYRxWAAQAG9gAKqv6+AwAAAABJRU5ErkJggg==" data-src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1vdmVybGFwLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6Mjg1fX19" alt="Diagram of NAT router translating IP addresses." width="285" height="99" border="0"> </picture> <noscript> "<img src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1vdmVybGFwLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6Mjg1fX19" width="285" alt="Diagram of NAT router translating IP addresses." border="0">" </noscript> <figcaption class="media-body-wrap bg-accent-3 p-4 w-max-full text-sm text-left"> The internal IP range (237.16.32.xx) is also a registered range used by another network. Therefore, the router is translating the addresses to avoid a potential conflict with another network. It will also translate the registered global IP addresses back to the unregistered local IP addresses when information is sent to the internal network. </figcaption> </figure> <p>The internal network is usually a LAN (Local Area Network), commonly referred to as the stub domain. A stub domain is a LAN that uses IP addresses internally. Most of the network traffic in a stub domain is local, so it doesn't travel outside the internal network. An internal IP address can be either registered or unregistered, and a stub domain can include it. Of course, any computers that use unregistered IP addresses must use NAT to communicate with the rest of the world.</p> <p>In the next section we'll look at the different ways NAT can be configured.</p> <div id="sidebar-page2-el22" class="sidebar-section lazyload bg-tan p-6 mb-6" > <div class="text-3xl lg:text-4xl mb-2 text-royal-blue">Thank You</div> <p class="text-base">Special thanks to <a href="http://www.cisco.com" class="ext-link"><b>Cisco</b></a> for its support in creating this article.</p> </div> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-14"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline6" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline6" class="ad-tag "></div></div> </div> </div> </div> </div> <div class="page-content relative clear-both" id="page-wrap3" x-data="{ pageVisible : true }"><a class="absolute left-0 top-n56" name="pt3"></a> <h2 class="page-title firePageview text-2xl mb-6 font-bold se" data-page-nbr="3" data-logged="false" data-page-url="nat3.htm"> <span class="flex cursor-pointer " @click="pageVisible = !pageVisible" aria-expanded="true" aria-controls="page3" role="button"> <svg class="mt-3 mr-3 print:hidden shrink-0 fill-current stroke-current transition ease-in-out duration-200 transform" :class="{ 'rotate-180' : !pageVisible, '' : pageVisible }" xmlns='http://www.w3.org/2000/svg' width='22' height='10' viewbox='0 0 28.396 13.211'><path d='M4398.8,5158.252l13.224,10.507,12.357-10.507' transform='translate(-4397.399 -5156.842)' fill='none' stroke-linecap='round' stroke-width='2'/></svg> <span>NAT Configuration</span> </span> </h2> <div class="page-body se" id="page3" data-slide="3" data-track-gtm="Content" x-show.transition="pageVisible === true"> <div class="editorial-sidebar mx-auto mb-6 md:float-left md:ml-0 md:mr-6 md:mb-4" style="max-width:290px;" data-track-gtm="Editorial Sidebar"> <figure class="fragment-media lead-image media-vertical" style="max-width: 290px;" > <picture class="lazyload"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1hZGRyZXNzLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6MjkwfSwidG9Gb3JtYXQiOiJhdmlmIn19" type="image/avif"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1hZGRyZXNzLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6MjkwfX19"> <img class=" bg-gray-light bg-opacity-50" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAJCAQAAACRI2S5AAAAEElEQVR42mNkIAAYRxWAAQAG9gAKqv6+AwAAAABJRU5ErkJggg==" data-src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1hZGRyZXNzLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6MjkwfX19" alt="Diagram of NAT router directing global and private IP networks." width="290" height="106" border="0"> </picture> <noscript> "<img src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1hZGRyZXNzLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6MjkwfX19" width="290" alt="Diagram of NAT router directing global and private IP networks." border="0">" </noscript> <figcaption class="media-body-wrap bg-accent-3 p-4 w-max-full text-sm text-left"> IP addresses have different designations based on whether they are on the private network (stub domain) or on the public network (Internet), and whether the traffic is incoming or outgoing. </figcaption> </figure> </div> <p>NAT can be configured in various ways. In the example below, the NAT router is configured to translate unregistered (inside, local) IP addresses, that reside on the private (inside) network, to registered IP addresses. This happens whenever a device on the inside with an unregistered address needs to communicate with the public (outside) network.</p> <div class="list"> <ul><li><span>An ISP assigns a range of IP addresses to your company. The assigned block of addresses are registered, unique IP addresses and are called inside global addresses. Unregistered, private IP addresses are split into two groups. One is a small group (outside local addresses) that will be used by the NAT routers. The other, much larger group, known as inside local IP addresses, will be used on the stub domain. The outside local addresses are used to translate the unique IP addresses, known as outside global addresses, of devices on the public network.</span></li></ul> </div> <div class="list"> <ul><li><span>Most computers on the stub domain communicate with each other using the inside local addresses.</span></li><li><span>Some computers on the stub domain communicate a lot outside the network. These computers have inside global addresses, which means that they do not require translation.</span></li><li><span>When a computer on the stub domain that has an inside local address wants to communicate outside the network, the packet goes to one of the NAT routers.</span></li><li><span>The NAT router checks the routing table to see if it has an entry for the destination address. If it does, the NAT device then translates the packet and creates an entry for it in the address translation table. If the destination address is not in the routing table, the packet is dropped.</span></li><li><span>Using an inside global address, the router sends the packet on to its destination.</span></li><li><span>A computer on the public network sends a packet to the private network. The source address on the packet is an outside global address. The destination address is an inside global address.</span></li><li><span>The NAT router looks at the address translation table and determines that the destination IP address is in there, mapped to a computer on the stub domain.</span></li><li><span>The NAT router translates the inside global address of the packet to the inside local address, and sends it to the destination computer.</span></li></ul> </div> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-99"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline7" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline7" class="ad-tag h-max-250"></div></div> </div> </div> </div> </div> <div class="page-content relative clear-both" id="page-wrap4" x-data="{ pageVisible : true }"><a class="absolute left-0 top-n56" name="pt4"></a> <h2 class="page-title firePageview text-2xl mb-6 font-bold se" data-page-nbr="4" data-logged="false" data-page-url="nat4.htm"> <span class="flex cursor-pointer " @click="pageVisible = !pageVisible" aria-expanded="true" aria-controls="page4" role="button"> <svg class="mt-3 mr-3 print:hidden shrink-0 fill-current stroke-current transition ease-in-out duration-200 transform" :class="{ 'rotate-180' : !pageVisible, '' : pageVisible }" xmlns='http://www.w3.org/2000/svg' width='22' height='10' viewbox='0 0 28.396 13.211'><path d='M4398.8,5158.252l13.224,10.507,12.357-10.507' transform='translate(-4397.399 -5156.842)' fill='none' stroke-linecap='round' stroke-width='2'/></svg> <span>The Connections of a NAT Overload</span> </span> </h2> <div class="page-body se" id="page4" data-slide="4" data-track-gtm="Content" x-show.transition="pageVisible === true"> <p>NAT overloading utilizes a feature of the <a href="https://computer.howstuffworks.com/web-server.htm">TCP/IP protocol stack</a>, multiplexing, that allows a computer to maintain several concurrent connections with a remote computer (or computers) using different TCP or UDP ports. An IP packet has a header that contains the following information:</p> <div class="list"> <ul><li><span><b>Source Address</b> - The IP address of the originating computer, such as 201.3.83.132</span></li><li><span><b>Source Port</b> - The TCP or UDP port number assigned by the originating computer for this packet, such as Port 1080</span></li><li><span><b>Destination Address</b> - The IP address of the receiving computer, such as 145.51.18.223</span></li><li><span><b>Destination Port</b> - The TCP or UDP port number that the originating computer is asking the receiving computer to open, such as Port 3021</span></li></ul> </div> <p>The addresses specify the two machines at each end, while the port numbers ensure that the connection between the two computers has a unique identifier. The combination of these four port numbers defines a single TCP/IP connection. Each port number uses 16 bits, which means that there are a possible 65,536 (2<sup>16</sup>) values. Realistically, since different manufacturers map the ports in slightly different ways, you can expect to have about 4,000 ports available.</p> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-99"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline9" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline9" class="ad-tag h-max-250"></div></div> </div> </div> </div> </div> <div class="page-content relative clear-both" id="page-wrap5" x-data="{ pageVisible : true }"><a class="absolute left-0 top-n56" name="pt5"></a> <h2 class="page-title firePageview text-2xl mb-6 font-bold se" data-page-nbr="5" data-logged="false" data-page-url="nat5.htm"> <span class="flex cursor-pointer " @click="pageVisible = !pageVisible" aria-expanded="true" aria-controls="page5" role="button"> <svg class="mt-3 mr-3 print:hidden shrink-0 fill-current stroke-current transition ease-in-out duration-200 transform" :class="{ 'rotate-180' : !pageVisible, '' : pageVisible }" xmlns='http://www.w3.org/2000/svg' width='22' height='10' viewbox='0 0 28.396 13.211'><path d='M4398.8,5158.252l13.224,10.507,12.357-10.507' transform='translate(-4397.399 -5156.842)' fill='none' stroke-linecap='round' stroke-width='2'/></svg> <span>Dynamic Network Address Translation</span> </span> </h2> <div class="page-body se" id="page5" data-slide="5" data-track-gtm="Content" x-show.transition="pageVisible === true"> <p>Here's how dynamic NAT works:</p> <div class="list"> <ul><li><span>An internal network (stub domain) has been set up with IP addresses that were not specifically allocated to that company by IANA (<a href="http://www.iana.org" class="ext-link">Internet Assigned Numbers Authority</a>), the global authority that hands out IP addresses. These addresses should be considered non-routable since they are not unique.</span></li><li><span>The company sets up a NAT-enabled router. The router has a range of unique IP addresses given to the company by IANA.</span></li><li><span>A computer on the stub domain attempts to connect to a computer outside the network, such as a web server.</span></li><li><span>The router receives the packet from the computer on the stub domain.</span></li><li><span>The router saves the computer's non-routable IP address to an address translation table. The router replaces the sending computer's non-routable IP address with the first available IP address out of the range of unique IP addresses. The translation table now has a mapping of the computer's non-routable IP address matched with the one of the unique IP addresses.</span></li><li><span>When a packet comes back from the destination computer, the router checks the destination address on the packet. It then looks in the address translation table to see which computer on the stub domain the packet belongs to. It changes the destination address to the one saved in the address translation table and sends it to that computer. If it doesn't find a match in the table, it drops the packet.</span></li><li><span>The computer receives the packet from the router. The process repeats as long as the computer is communicating with the external system.</span></li></ul> </div> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-99"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline11" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline11" class="ad-tag h-max-250"></div></div> </div> </div> </div> </div> <div class="page-content relative clear-both" id="page-wrap6" x-data="{ pageVisible : true }"><a class="absolute left-0 top-n56" name="pt6"></a> <h2 class="page-title firePageview text-2xl mb-6 font-bold se" data-page-nbr="6" data-logged="false" data-page-url="nat6.htm"> <span class="flex cursor-pointer " @click="pageVisible = !pageVisible" aria-expanded="true" aria-controls="page6" role="button"> <svg class="mt-3 mr-3 print:hidden shrink-0 fill-current stroke-current transition ease-in-out duration-200 transform" :class="{ 'rotate-180' : !pageVisible, '' : pageVisible }" xmlns='http://www.w3.org/2000/svg' width='22' height='10' viewbox='0 0 28.396 13.211'><path d='M4398.8,5158.252l13.224,10.507,12.357-10.507' transform='translate(-4397.399 -5156.842)' fill='none' stroke-linecap='round' stroke-width='2'/></svg> <span>How NAT Overloading Works</span> </span> </h2> <div class="page-body se" id="page6" data-slide="6" data-track-gtm="Content" x-show.transition="pageVisible === true"> <p>Here's how overloading works:</p> <div class="list"> <ul><li><span>An internal network (stub domain) has been set up with non-routable IP addresses that were not specifically allocated to that company by IANA.</span></li><li><span>The company sets up a NAT-enabled router. The router has a unique IP address given to the company by IANA.</span></li><li><span>A computer on the stub domain attempts to connect to a computer outside the network, such as a Web server.</span></li><li><span>The router receives the packet from the computer on the stub domain.</span></li><li><span>The router saves the computer's non-routable IP address and port number to an address translation table. The router replaces the sending computer's non-routable IP address with the router's IP address. The router replaces the sending computer's source port with the port number that matches where the router saved the sending computer's address information in the address translation table. The translation table now has a mapping of the computer's non-routable IP address and port number along with the router's IP address.</span></li><li><span>When a packet comes back from the destination computer, the router checks the destination port on the packet. It then looks in the address translation table to see which computer on the stub domain the packet belongs to. It changes the destination address and destination port to the ones saved in the address translation table and sends it to that computer.</span></li><li><span>The computer receives the packet from the router. The process repeats as long as the computer is communicating with the external system.</span></li><li><span>Since the NAT router now has the computer's source address and source port saved to the address translation table, it will continue to use that same port number for the duration of the connection. A timer is reset each time the router accesses an entry in the table. If the entry is not accessed again before the timer expires, the entry is removed from the table.</span></li></ul> </div> <p>In the next section we'll look at the organization of stub domains.</p> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-99"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline13" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline13" class="ad-tag h-max-250"></div></div> </div> </div> </div> </div> <div class="page-content relative clear-both" id="page-wrap7" x-data="{ pageVisible : true }"><a class="absolute left-0 top-n56" name="pt7"></a> <h2 class="page-title firePageview text-2xl mb-6 font-bold se" data-page-nbr="7" data-logged="false" data-page-url="nat7.htm"> <span class="flex cursor-pointer " @click="pageVisible = !pageVisible" aria-expanded="true" aria-controls="page7" role="button"> <svg class="mt-3 mr-3 print:hidden shrink-0 fill-current stroke-current transition ease-in-out duration-200 transform" :class="{ 'rotate-180' : !pageVisible, '' : pageVisible }" xmlns='http://www.w3.org/2000/svg' width='22' height='10' viewbox='0 0 28.396 13.211'><path d='M4398.8,5158.252l13.224,10.507,12.357-10.507' transform='translate(-4397.399 -5156.842)' fill='none' stroke-linecap='round' stroke-width='2'/></svg> <span>Stub Domains</span> </span> </h2> <div class="page-body se" id="page7" data-slide="7" data-track-gtm="Content" x-show.transition="pageVisible === true"> <p>Look below to see how the computers on a stub domain might appear to external networks.</p> <h3 class="font-bold text-xl mb-2">Source Computer A</h3> <p>IP Address: 192.168.32.10</p> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-99"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline15" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline15" class="ad-tag h-max-250"></div></div> </div> </div> <p>Computer Port: 400</p> <p>NAT Router IP Address: 215.37.32.203</p> <p>NAT Router Assigned Port Number: 1</p> <h3 class="font-bold text-xl mb-2">Source Computer B</h3> <p>IP Address: 192.168.32.13</p> <p>Computer Port: 50</p> <p>NAT Router IP Address: 215.37.32.203</p> <p>NAT Router Assigned Port Number: 2</p> <h3 class="font-bold text-xl mb-2">Source Computer C</h3> <p>IP Address: 192.168.32.15</p> <p>Computer Port: 3750</p> <p>NAT Router IP Address: 215.37.32.203</p> <p>NAT Router Assigned Port Number: 3</p> <h3 class="font-bold text-xl mb-2">Source Computer D</h3> <p>IP Address: 192.168.32.18</p> <p>Computer Port: 206</p> <p>NAT Router IP Address: 215.37.32.203</p> <p>NAT Router Assigned Port Number: 4</p> <p>As you can see, the NAT router stores the IP address and port number of each computer. It then replaces the IP address with its own registered IP address and the port number corresponding to the location, in the table, of the entry for that packet's source computer. So any external network sees the NAT router's IP address and the port number assigned by the router as the source-computer information on each packet.</p> <p>You can still have some computers on the stub domain that use dedicated IP addresses. You can create an access list of IP addresses that tells the router which computers on the network require NAT. All other IP addresses will pass through untranslated.</p> <p>The number of simultaneous translations that a router will support are determined mainly by the amount of DRAM (Dynamic Random Access Memory) it has. But since a typical entry in the address-translation table only takes about 160 bytes, a router with 4 MB of DRAM could theoretically process 26,214 simultaneous translations, which is more than enough for most applications.</p> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-14"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline16" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline16" class="ad-tag "></div></div> </div> </div> </div> </div> <div class="page-content relative clear-both" id="page-wrap8" x-data="{ pageVisible : true }"><a class="absolute left-0 top-n56" name="pt8"></a> <h2 class="page-title firePageview text-2xl mb-6 font-bold se" data-page-nbr="8" data-logged="false" data-page-url="nat8.htm"> <span class="flex cursor-pointer " @click="pageVisible = !pageVisible" aria-expanded="true" aria-controls="page8" role="button"> <svg class="mt-3 mr-3 print:hidden shrink-0 fill-current stroke-current transition ease-in-out duration-200 transform" :class="{ 'rotate-180' : !pageVisible, '' : pageVisible }" xmlns='http://www.w3.org/2000/svg' width='22' height='10' viewbox='0 0 28.396 13.211'><path d='M4398.8,5158.252l13.224,10.507,12.357-10.507' transform='translate(-4397.399 -5156.842)' fill='none' stroke-linecap='round' stroke-width='2'/></svg> <span>How IP Address Information is Classified</span> </span> </h2> <div class="page-body se" id="page8" data-slide="8" data-track-gtm="Content" x-show.transition="pageVisible === true"> <p>IANA has set aside specific ranges of IP addresses for use as non-routable, internal network addresses. These addresses are considered unregistered (for more information check out <a href="http://www.ietf.org/rfc/rfc1918.txt" class="ext-link">RFC 1918: Address Allocation for Private Internets</a>, which defines these address ranges). No company or agency can claim ownership of unregistered addresses or use them on public computers.</p> <p>Routers are designed to discard (instead of forward) unregistered addresses. What this means is that a packet from a computer with an unregistered address could reach a registered destination computer, but the reply would be discarded by the first router it came to.</p> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-99"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline17" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline17" class="ad-tag h-max-250"></div></div> </div> </div> <p>There is a range for each of the three classes of IP addresses used for networking:</p> <div class="list"> <ul><li><span>Range 1: Class A - 10.0.0.0 through 10.255.255.255</span></li><li><span>Range 2: Class B - 172.16.0.0 through 172.31.255.255</span></li><li><span>Range 3: Class C - 192.168.0.0 through 192.168.255.255</span></li></ul> </div> <p>Although each range is in a different class, your are not required to use any particular range for your internal network. It is a good practice, though, because it greatly diminishes the chance of an IP address conflict.</p> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-14"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline18" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline18" class="ad-tag "></div></div> </div> </div> </div> </div> <div class="page-content relative clear-both" id="page-wrap9" x-data="{ pageVisible : true }"><a class="absolute left-0 top-n56" name="pt9"></a> <h2 class="page-title firePageview text-2xl mb-6 font-bold se" data-page-nbr="9" data-logged="false" data-page-url="nat9.htm"> <span class="flex cursor-pointer " @click="pageVisible = !pageVisible" aria-expanded="true" aria-controls="page9" role="button"> <svg class="mt-3 mr-3 print:hidden shrink-0 fill-current stroke-current transition ease-in-out duration-200 transform" :class="{ 'rotate-180' : !pageVisible, '' : pageVisible }" xmlns='http://www.w3.org/2000/svg' width='22' height='10' viewbox='0 0 28.396 13.211'><path d='M4398.8,5158.252l13.224,10.507,12.357-10.507' transform='translate(-4397.399 -5156.842)' fill='none' stroke-linecap='round' stroke-width='2'/></svg> <span>Security and Administration</span> </span> </h2> <div class="page-body se" id="page9" data-slide="9" data-track-gtm="Content" x-show.transition="pageVisible === true"> <div class="editorial-sidebar mx-auto mb-6 md:float-left md:ml-0 md:mr-6 md:mb-4" style="max-width:290px;" data-track-gtm="Editorial Sidebar"> <figure class="fragment-media lead-image media-vertical" style="max-width: 290px;" > <picture class="lazyload"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1zdGF0aWMuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyOTB9LCJ0b0Zvcm1hdCI6ImF2aWYifX0=" type="image/avif"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1zdGF0aWMuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyOTB9fX0="> <img class=" bg-gray-light bg-opacity-50" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAJCAQAAACRI2S5AAAAEElEQVR42mNkIAAYRxWAAQAG9gAKqv6+AwAAAABJRU5ErkJggg==" data-src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1zdGF0aWMuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyOTB9fX0=" alt="Diagram showing Static NAT directing traffic toward a public network." width="290" height="94" border="0"> </picture> <noscript> "<img src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1zdGF0aWMuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyOTB9fX0=" width="290" alt="Diagram showing Static NAT directing traffic toward a public network." border="0">" </noscript> <figcaption class="media-body-wrap bg-accent-3 p-4 w-max-full text-sm text-left"> Static NAT (inbound mapping) allows a computer on the stub domain to maintain a specific address when communicating with devices outside the network. </figcaption> </figure> </div> <p>Implementing dynamic NAT automatically creates a firewall between your internal network and outside networks, or between your internal network and the Internet. NAT only allows connections that originate inside the stub domain.</p> <p>Essentially, this means that a computer on an external network cannot connect to your computer unless your computer has initiated the contact. You can browse the Internet and connect to a site, and even download a file; but somebody else cannot latch onto your IP address and use it to connect to a port on your computer.</p> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-99"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline19" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline19" class="ad-tag h-max-250"></div></div> </div> </div> <p>In specific circumstances, Static NAT, also called inbound mapping, allows external devices to initiate connections to computers on the stub domain. For instance, if you wish to go from an inside global IP address to a specific inside local IP address that is assigned to your web server, Static NAT would enable the connection.</p> <p>Some NAT routers provide for extensive filtering and traffic logging. Filtering allows your company to control what type of sites employees visit on the web, preventing them from viewing questionable material. You can use traffic logging to create a log file of what sites are visited and generate various reports from it.</p> <div class="sticky-container ad-container print:hidden md:hidden h-min-400 mb-6"> <div class="sticky top-14"> <p class="ad-disclaimer clear-both text-xs text-center mb-1 md:hidden">Advertisement</p> <div id="ad-wrap-mobinline20" class="print:hidden md:hidden ad-inline ad-mobinline bg-gray w-max-full h-min-250"><div id="ad-div-mobinline20" class="ad-tag "></div></div> </div> </div> </div> </div> <div class="page-content relative clear-both" id="page-wrap10" x-data="{ pageVisible : true }"><a class="absolute left-0 top-n56" name="pt10"></a> <h2 class="page-title firePageview text-2xl mb-6 font-bold se" data-page-nbr="10" data-logged="false" data-page-url="nat10.htm"> <span class="flex cursor-pointer " @click="pageVisible = !pageVisible" aria-expanded="true" aria-controls="page10" role="button"> <svg class="mt-3 mr-3 print:hidden shrink-0 fill-current stroke-current transition ease-in-out duration-200 transform" :class="{ 'rotate-180' : !pageVisible, '' : pageVisible }" xmlns='http://www.w3.org/2000/svg' width='22' height='10' viewbox='0 0 28.396 13.211'><path d='M4398.8,5158.252l13.224,10.507,12.357-10.507' transform='translate(-4397.399 -5156.842)' fill='none' stroke-linecap='round' stroke-width='2'/></svg> <span>A Note on Proxy Servers</span> </span> </h2> <div class="page-body se" id="page10" data-slide="10" data-track-gtm="Content" x-show.transition="pageVisible === true"> <p>NAT is sometimes confused with proxy servers, but there are definite differences between them. NAT is transparent to the source and to destination computers. Neither one realizes that it is dealing with a third device. But a proxy server is not transparent.</p> <p>The source computer knows that it is making a request to the proxy server and must be configured to do so. The destination computer thinks that the proxy server <b>IS</b> the source computer, and deals with it directly. Also, proxy servers usually work at layer 4 (transport) of the <a href="https://computer.howstuffworks.com/osi.htm">OSI Reference Model</a> or higher, while NAT is a layer 3 (network) protocol. Working at a higher layer makes proxy servers slower than NAT devices in most cases.</p> <figure class="fragment-media clear-both w-max-full mx-auto mb-6 md:float-left md:ml-0 md:mr-6 md:mb-4" style="max-width: 285px;" style="max-width: 285px;"> <picture class="lazyload"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1vc2kuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyODV9LCJ0b0Zvcm1hdCI6ImF2aWYifX0=" type="image/avif"> <source " data-srcset="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1vc2kuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyODV9fX0="> <img class=" bg-gray-light bg-opacity-50" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAJCAQAAACRI2S5AAAAEElEQVR42mNkIAAYRxWAAQAG9gAKqv6+AwAAAABJRU5ErkJggg==" data-src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1vc2kuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyODV9fX0=" alt="Diagram of OSI reference model, showing NAT at layer 3 (the Network layer)." width="285" height="325" border="0"> </picture> <noscript> "<img src="https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL25hdC1vc2kuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjoyODV9fX0=" width="285" alt="Diagram of OSI reference model, showing NAT at layer 3 (the Network layer)." border="0">" </noscript> <figcaption class="media-body-wrap bg-accent-3 p-4 w-max-full text-sm text-left"> NAT operates at the Network layer (layer 3) of the OSI Reference Model -- this is the layer that routers work at. </figcaption> </figure> <p>A real benefit of NAT is apparent in network administration. For example, you can move your Web server or FTP server to another host computer without having to worry about broken links. Simply change the inbound mapping at the router to reflect the new host. You can also make changes to your internal network easily, because the only external IP address either belongs to the router or comes from a pool of global addresses.</p> <p>NAT and DHCP (dynamic host configuration protocol ) are a natural fit. You can choose a range of unregistered IP addresses for your stub domain and have the DHCP server dole them out as necessary. It also makes it much easier to scale up your network as your needs grow. You don't have to request more IP addresses from IANA. Instead, you can just increase the range of available IP addresses configured in DHCP to immediately have room for additional computers on your network.</p> </div> </div> <div class="page-content relative clear-both" id="page-wrap11" x-data="{ pageVisible : true }"><a class="absolute left-0 top-n56" name="pt11"></a> <h2 class="page-title firePageview text-2xl mb-6 font-bold se" data-page-nbr="11" data-logged="false" data-page-url="nat11.htm"> <span class="flex cursor-pointer " @click="pageVisible = !pageVisible" aria-expanded="true" aria-controls="page11" role="button"> <svg class="mt-3 mr-3 print:hidden shrink-0 fill-current stroke-current transition ease-in-out duration-200 transform" :class="{ 'rotate-180' : !pageVisible, '' : pageVisible }" xmlns='http://www.w3.org/2000/svg' width='22' height='10' viewbox='0 0 28.396 13.211'><path d='M4398.8,5158.252l13.224,10.507,12.357-10.507' transform='translate(-4397.399 -5156.842)' fill='none' stroke-linecap='round' stroke-width='2'/></svg> <span>Multi-homing</span> </span> </h2> <div class="page-body se" id="page11" data-slide="11" data-track-gtm="Content" x-show.transition="pageVisible === true"> <p>As businesses rely more and more on the Internet, having multiple points of connection to the Internet is fast becoming an integral part of their network strategy. Multiple connections, known as multi-homing, reduces the chance of a potentially catastrophic shutdown if one of the connections should fail.</p> <p>In addition to maintaining a reliable connection, multi-homing allows a company to perform load-balancing by lowering the number of computers connecting to the Internet through any single connection. Distributing the load through multiple connections optimizes the performance and can significantly decrease wait times.</p> <p>Multi-homed networks are often connected to several different ISPs (Internet Service Providers). Each ISP assigns an IP address (or range of IP addresses) to the company. Routers use BGP (Border Gateway Protocol), a part of the TCP/IP protocol suite, to route between networks using different protocols. In a multi-homed network, the router utilizes IBGP (Internal Border Gateway Protocol) on the stub domain side, and EBGP (External Border Gateway Protocol) to communicate with other routers.</p> <p>Multi-homing really makes a difference if one of the connections to an ISP fails. As soon as the router assigned to connect to that ISP determines that the connection is down, it will reroute all data through one of the other routers.</p> <p>NAT can be used to facilitate scalable routing for multi-homed, multi-provider connectivity. For more on multi-homing, see <a href="http://www.cisco.com/warp/public/cc/pd/iosw/ioft/ionetn/tech/emios_wp.htm" class="ext-link">Cisco: Enabling Enterprise Multihoming</a>.</p> <p>For lots more information on NAT and related topics, check out the links on the next page.</p> </div> </div> <style> .faq { color: #707070; font-size: 1.125rem; line-height: 1.75; border-color: #a2d4d3; } .faq h3 { display: flex; align-items: center; font-weight: bold; color: #2B2B5C; font-size: 1.25rem; margin-bottom: 1.25rem; } .faq h3::before { content: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='26' height='26' viewBox='0 0 26 26'%3E%3Cpath d='M27.7,8.2H25.1V19.9H8.2v2.6a1.3,1.3,0,0,0,1.3,1.3H23.8L29,29V9.5A1.3,1.3,0,0,0,27.7,8.2ZM22.5,16V4.3A1.3,1.3,0,0,0,21.2,3H4.3A1.3,1.3,0,0,0,3,4.3V22.5l5.2-5.2h13A1.3,1.3,0,0,0,22.5,16Z' transform='translate(-3 -3)' fill='%2336a2a0'/%3E%3C/svg%3E"); color: #36A2A0; width: 26px; height: 26px; margin-right: 12px; } .faq h3::after { content: ''; height: 1px; background-color: #36A2A0; flex: 1 1 0%; margin-left: 12px; } .faq h5 { font-weight: bold; margin-bottom: 0.75rem; } .faq p { margin-bottom: 0.75rem; font-size: 1rem; line-height: 1.25; } .faq .questionContainer { margin-bottom: 1.5rem; } .faq .questionContainer:last-child { margin-bottom: 0; } </style> <div class="faq p-6 mt-6 mb-6 clear-both border"> <h3>Network Address Translation (NAT) FAQ</h3> <div class="questionContainer" itemscope itemprop="mainEntity" itemtype="https://schema.org/Question"> <h5 class="question" itemprop="name">What is a Network Address Translation?</h5> <div class="answerContainer" itemscope itemprop="acceptedAnswer" itemtype="https://schema.org/Answer"> <div class="answer" itemprop="text">A Network Address Translation or NAT is a mapping method of providing internet connection to local servers and hosts. In NAT, you take several local IPs and map them to one single global IP to transmit information across a routing device.</div> </div> </div> <div class="questionContainer" itemscope itemprop="mainEntity" itemtype="https://schema.org/Question"> <h5 class="question" itemprop="name">Does NAT affect Internet speed?</h5> <div class="answerContainer" itemscope itemprop="acceptedAnswer" itemtype="https://schema.org/Answer"> <div class="answer" itemprop="text">NAT only affects a little bit of your internet speed. It is barely noticeable if youâ€™re using a reasonable router for translating your IPs. </div> </div> </div> <div class="questionContainer" itemscope itemprop="mainEntity" itemtype="https://schema.org/Question"> <h5 class="question" itemprop="name">What benefits are offered by Network Address Translation?</h5> <div class="answerContainer" itemscope itemprop="acceptedAnswer" itemtype="https://schema.org/Answer"> <div class="answer" itemprop="text">With NAT enabled, it is easier to re-use your personal IP addresses with extra security. Moreover, NAT allows you to keep your external and internal IP addresses private and secure. You can also save the memory of your IP address by connecting several hosts via the internet using only a few external IPs.</div> </div> </div> <div class="questionContainer" itemscope itemprop="mainEntity" itemtype="https://schema.org/Question"> <h5 class="question" itemprop="name">What is difference between NAT and PAT?</h5> <div class="answerContainer" itemscope itemprop="acceptedAnswer" itemtype="https://schema.org/Answer"> <div class="answer" itemprop="text">NAT stands for Network Address Translation while PAT stands for Port Address Translation. As the names suggest, both NAT and PAT are used to translate private IPs into public IPs to save space and connect multiple devices. The difference is that PAT uses port numbers to map IP addresses whereas NAT doesnâ€™t.</div> </div> </div> <div class="questionContainer" itemscope itemprop="mainEntity" itemtype="https://schema.org/Question"> <h5 class="question" itemprop="name">How many types of NAT exist?</h5> <div class="answerContainer" itemscope itemprop="acceptedAnswer" itemtype="https://schema.org/Answer"> <div class="answer" itemprop="text">There are many forms of NAT. Static NAT maps an unregistered IP address to a registered IP address on a one-to-one basis; Dynamic NAT maps an unregistered IP address to a registered IP address from a group of registered IP addresses; Overloading maps multiple unregistered IP addresses to a single registered IP address by using different ports; Overlapping happens when a device on one network is assigned an IP address on the same subnet as another device on the internet or external network.</div> </div> </div> â€ƒ </div> <div class="page-content relative clear-both" id="page-wrap12" x-data="{ pageVisible : true }"><a class="absolute left-0 top-n56" name="pt12"></a> <h2 class="page-title firePageview text-2xl mb-6 font-bold" data-page-nbr="12" data-logged="false" data-page-url="nat12.htm"> <span class="flex cursor-pointer " @click="pageVisible = !pageVisible" aria-expanded="true" aria-controls="page12" role="button"> <svg class="mt-3 mr-3 print:hidden shrink-0 fill-current stroke-current transition ease-in-out 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href="https://computer.howstuffworks.com/home-network.htm">How Home Networking Works</a></span></li><li><span><a href="https://computer.howstuffworks.com/osi.htm">How OSI Works</a></span></li></ul> </div> <h3 class="font-bold text-xl mb-2">More Great Links</h3> <div class="list"> <ul><li><span><a href="http://www.vicomsoft.com/knowledge/reference/nat.html" class="ext-link">Network Address Translation FAQ</a></span></li><li><span><a href="http://www.netsizer.com/" class="ext-link">Netsizer: Realtime Internet Growth</a></span></li><li><span>Cisco: Network Address Translation</span></li><li><span><a href="http://www.safety.net/nattech.html" class="ext-link">NAT Technical Discussion</a></span></li><li><span><a href="http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/np1_c/1cprt2/1cipadr.htm" class="ext-link">Cisco: Configuring IP Addressing</a></span></li><li><span><a href="http://www.cisco.com/warp/public/556/3.html" class="ext-link">Cisco: NAT Overlapping</a></span></li><li><span><a href="http://www.cisco.com/warp/public/556/5.html" class="ext-link">Cisco: NAT Order of Operation</a></span></li><li><span><a href="http://www.cisco.com/warp/public/759/ipj_3-4/ipj_3-4_nat.html" class="ext-link">IP Journal: The Trouble With NAT</a></span></li><li><span><a href="http://www.cisco.com/warp/public/cc/pd/iosw/ioft/ionetn/tech/emios_wp.htm" class="ext-link">Cisco: Enabling Enterprise Multihoming</a></span></li><li><span><a href="http://www.faqs.org/rfcs/rfc1631.html" class="ext-link">RFC 1631: The IP Network Address Translator (NAT)</a></span></li><li><span><a href="http://www.ietf.org/rfc/rfc1918.txt" class="ext-link">RFC 1918: Address Allocation for Private Internets</a></span></li></ul> </div> </div> </div> </article> <div id="modal-citation-modal" x-cloak x-data="{ open: false }" x-show="open" @citation-modal.window="open = !open" class="fixed z-10 inset-0 overflow-y-auto" aria-modal="true"> <div class="flex items-end 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