Andrew Yager's Blog - There is a difference between NAT and PAT!

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There is a difference between NAT and PAT!

Written by Andrew Yager

Sunday, 22 January 2006

It sometimes bugs me... like right now... that when I am trying to find information about a particular technology people have universally used the wrong term to describe a commonly used feature. Take NAT (Network Address Translation) and PAT (Port Address Translation). NAT allows you to translate or map one IP address onto another single ip address. PAT on the other hand is what is most commonly referred to as NAT. In a PAT system you have a single or group of public IP addresses that are translated to multiple internal ip addresses by mapping the TCP/UDP ports to different ports. This means that by using some "magic" on a router or server you can get around problems that you might have with two web browsers sending a request out the same port.

But why do people get confused, and why do I care?

This is the question that I propose to begin answering. Essentially, the problem is that some (indeed most) vendors have taken to using the wrong terms because users didn't know better. In the internet's younger days, people would buy NAT enabled software packages that did port address translation. With the advent of broadband and the introduction of consumer ADSL and Cable routers, someone designed an interface which called PAT NAT. In fairness to these people, PAT is a kind of NAT - you are translating a single outside IP address to inside IP addresses... but they are still fundamentally different concepts!

The down side of having both terms used for the same thing is that when I am after some information about NAT - that is TRUE NAT, without Port Address Translation, but merely forwarding requests with the IP headers changed (which is necessary to make NAT work) is that I can't find any information on it's implementation. Even for my Cisco 1700 series router. I haven't looked a lot, and decided to rant before I got too far, but I mean really... is it that hard?

I'm calling all of the internet to correct your terms. Make sure your acronmyns mean what you think they mean. And don't confuse NAT and PAT any longer. Reform!

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