IP Forwarding versus NAT

5/29/2009

I could not find a definitive answer to this. After much frustration and by accident, I've found the answer to what the difference between these two are.

At first it appeared that NAT was just IP forwrading plus reverse support (ie full cone NAT), and UDP tunneling. But for my needs, I was only interested in what the difference was for outbound connections. I concluded (incorrectly) that for outbound connections, they were in fact the same and that IP forwarding was simply one component of NAT.

However there is at least one key difference. NAT changes the MAC address of the outgoing packets so they appear to come from the NAT machine, while IP forwarding just throws the packets down the line with the orginal MAC address in tact. That is, it acts as a router.

In most cases this is not a big deal, however if your upline provider does MAC filtering the difference is qutie important. Many ISP's today lock your connection to your internet device or last computer that made a DHCP request.

Configuring NAT on Windows

NAT is easily configured on Windows by using Internet Connection Sharing (ICS). However this requries you to use a fixed IP subnet which often can be problematic as it is the most common private subnet used by other networks and devices.

On Windows Server you can instead configure RRAS manually and use it instead of ICS. Apparently it is available in XP as well if you use the command line. I wish I had discovered this a couple of years ago. However do not get too excited, as apparently the routing command has been removed from Vista.

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