Introduction

Network Address Translation, defined by RFC 1631, is becoming very popular in today's networks as it's supported by almost every operating system, firewall appliance and application. NAT was born thanks to the fast depletion of public IP Addresses, in other words real IP Addresses that can only exist on the Internet.

As IP Addresses are 32 Bit, in theory we could have upto 4,294,967,296 IP Addresses (that's 2^32) ! In practice though the number is a lot smaller, somewhere around 3.2 billion, because of the way the IP Addresses are separated into Classes (Class A, B , C e.t.c) and the need to set aside special addresses for multicasting (also known as Class D), broadcasting and other functions.

So What's Covered ?

As with most cool networking topics, it's impossible to cover NAT on one page and if you happen to find another site that covers NAT in one page, I assure you you're missing out on a lot of important information, so stick to Firewall.cx :)

With all this in mind, I've split NAT into 6 sections. Each section deals with a particular NAT mode or NAT topic, giving you an in-depth look on how each NAT mode works using a few examples, and its advantages over the rest available NAT modes. The information provided has been carefully selected and written to make sure it covers all ranges of user levels, meaning from intermediate to advanced.

Section 1: NAT Concepts. A good introduction to NAT followed by its basic functions, how it works and which devices in a network usually implement NAT. Simple, clear and colourful diagrams will ensure you grasp this concept without any trouble.

Section 2: NAT Table. This section will introduce the NAT Table, which is the heart of NAT. Here you will learn the purpose of the NAT table, where it's stored along with a lot of other interesting information.

Section 3: Static NAT Mode. Learn what Static NAT is and how it functions. Two pages of detailed diagrams, well thought examples and their analysis along with other rich information ensures you will learn everything there is about Static NAT.

Section 4: Dynamic NAT Mode. Learn what Dynamic NAT is and how it functions. Simple diagrams are available to help you understand how Dynamic NAT works and what its advantages are over Static NAT. Dynamic NAT is analysed over two pages using examples and step by step analysis, ensuring to capture all the required information and answer every question you might have.

Section 5: NAT Overload Mode. Also known as IP Masquerading (in the Linux world), Port Address Translation (PAT) or Dynamic NAT with PAT. Discover the most common NAT mode for small networks. This NAT mode is used by most Internet sharing software. This section will help you understand how NAT Overload works and what its benefits are over the rest. Again, simple diagrams have been designed to make sure you grasp all this cool stuff :)

The type of NAT mode you choose to use, depends on your network resources, capabilities of your NAT-enabled device and, lastly, your needs. Together we will discover the power of NAT and understand why its become so popular.