How do I know if I'm behind a NAT router?

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Summary: NAT routers are a fundamental way to share an internet connection while protecting you at the same time. You may already have one. It's easy to check.

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The answer's not as obvious as a lot of people are thinking. Yes, much of the time a NAT router is an additional box ... a device that you plug your computer into that, in turn, plugs into your internet connection. And that box will typically say "router" on it.

But that's not the only way you can end up behind a router.

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As it turns out, many broadband modems also act as a router - and they won't say it on the outside. Knowing whether they're acting as a NAT router is important - you might be able to avoid buying an extra router you don't actually need, or you might be able to avoid installing firewall software you don't actually need.

Finding out is fairly simple.

My favorite way is to open a Command Prompt - that's usually done this way:

• Click on Start

• Click on All Programs

• Click on Accessories

• Click on Command Prompt

You'll probably get window similar to this one:

In that Window type "ipconfig" followed by the enter key, and you should get something like this:

Now, like me, you may have more than one network adapter. The one you probably want to pay attention to is the one labeled Local Area Connection, for wired connections, or Wireless Network Connection if you're running via a WiFi or other wireless connection.

The line you care about is this one:

IP Address. . . . . . . . . . . . : 192.168.1.2

That's your computer's IP address. If it begins with "192.168", as mine does, then you are almost certainly behind a NAT router. IP addresses that begin with "192.168" are actually invalid on the internet as they are specifically intended for non-internet, local networks. The router performs what's called "Network Address Translation", or NAT, to translate from your local IP address to your internet IP address as assigned by your ISP.

You may also see an IP address beginning with "10.", or some addresses beginning with "172." - these, too, may mean that you are behind a router of some sort, but these are typically NOT used by modems and consumer-grade routers. They're typically used for larger installations such as corporations, hotels or some ISPs.

So if your computer is connected directly to your broadband modem, and you have a "192.168." address, then your modem is acting as a NAT router. If you have some other address, then it's not, and you probably want to investigate installing a NAT router or firewall.

Related:

• Ask Leo! - Zone Alarm firewall: do I need it if I'm behind a NAT router?

• Ask Leo! - Do I need a firewall, and if so, what kind?

• Ask Leo! - What's the difference between a Hub, a Switch and a Router?

Article C2888 - January 4, 2007

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