Network Address Translation (NAT) Tutorial

Just what is Network Address Translation? Find out everything you want to know from this free NAT tutorial by Neil Briscoe from PC Network Advisor

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Network Address Translation is commonly known as NAT. In this article I’ll explain what NAT is and give a brief technical explanation of how it works.

Why do we need it? Well, it is one attempt at preserving the address space available for use on the Internet.

Internet Service Providers no longer use the A, B, and C classes of address. They allocate addresses in a classless manner. What that means is that, from their blocks of allocated addresses, they will allocate you just as many addresses as you need, and no more. This method is known as CIDR (pronounced Cider) which stands for Classless Internet Domain Routing. CIDR is, however, outside the scope of this article.

All you need to know about CIDR for the purposes of this article is that it means that, if you change ISPs, you’re going to have to change all of your IP addresses, unless you adhere to RFC 1918 in the first place. Believe you me, if you have even a medium- sized network, changing all the IP addresses on all of your machines requires a considerable investment in time and money, not to mention the fact that you will have to reboot many machines, which can mean things fail that might otherwise have held on a little longer.

On the other hand, if you have adhered to RFC 1918 you will only need to change the address on the external side of your router, and possibly any static HAT mappings you’ve used - I’ll explain that later.

IP Connections

Before we delve further into Network Address Translation we need to talk about how IP connections normally work.

There are 65, 535 ports - you can think of them as channels - for IP to choose from. (The number of ports available is determined by the maximum value you can get from the 16 bits allocated to the port number in the IP packet.)

The first 1023 of them are reserved. They’re deemed to be privileged ports and are reserved for what are known as the “Well Known Services” (WKS) such as telnet, ftp, gopher, www and so on. So they’re normally reserved for server processes for those types of protocol, and can’t be used by client processes.

The remaining ports can be used for anything. Some of these are anyway used by various server processes - and some are becoming “Well Known” but until IANA grants them a number that use is merely an unofficial convention.

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