While Network Address Translation, or NAT, doesn’t appear on the exam outline, I have heard from several sources that it appears on the composite exam. It’s also one of those topics that I’d expect to be tested on in the simulator.

NAT is conceptually pretty easy — you’re manipulating addresses and possibly ports in order to conceal addresses. If you had an RFC1918 addressed host inside and needed the Internet to connect to it, you’d want to translate the destination address somewhere. However, Cisco has come out with some rather confusing terms, such as inside global and outside local (and other combinations) to describe the scenarios and configurations.

Inside, outside, local, and global

How NAT Works describes the need for NAT and goes into detail on the internals. Basically there are two different types — dynamic and static. In a static NAT, the same address is always substituted for the same host. For example, if you’re allowing an internal web server to be accessed from the inside, then you need to static NAT the connection in. A dynamic NAT allows these substitutions to change. It doesn’t matter what IP the world sees if you are web browsing. Overloading allows multiple people to share the same external address. Overlapping means that two networks use the same address space, and NAT is used to resolve the problem.

The first thing to do is establish the inside and the outside. The inside is usually your internal network, the outside is the Internet or a third party. Connections initiated from the inside are then inside connections, and incoming connections are outside connections.

Now, put yourself on the inside network. An address that you see is a local address. An address that the Internet sees is a global address.

For example, say you wanted to hide your privately addressed PC’s web surfing behind an Internet accessible address. The PC’s address is a inside local address. The address that the Internet sees is the inside global. To go back to our internal web server, the internal address is the outside local address, the address the Internet uses to access it is the outside global address.

More information on NAT: Local and Global Definitions

A simple example is coming…

Some more links from CCO:

NAT Order of Operation is important.

Verifying NAT Operation and Basic NAT Troubleshooting

Configuring Network Address Translation: Getting Started

References

1. How NAT Works
2. NAT: Local and Global Definitions
3. NAT Order of Operation
4. Verifying NAT Operation and Basic NAT Troubleshooting
5. Configuring Network Address Translation: Getting Started