Why doesn't my machine's IP address match what I'm told on the internet?

What you're seeing is the result of what's called a NAT router. NAT stands for "Network Address Translation". The router manages the IP addresses on a LAN, or Local Area Network, and then translates to the appropriate IP address it was assigned on the external network, usually the internet.

Let's back up and look at how IP addresses are assigned.

In most cases, internet addresses are assigned dynamically:

In this example, using a protocol called DHCP, for "Dynamic Host Configuration Protocol", when the computer connects to its ISP or network provider, it asks for an IP address, and that IP address is then assigned by the ISP. It's that IP address that the computer will be identified by on the internet. The IP address may change - the next time the computer connects it may be given a different address - hence this is called a "dynamic" IP address.

A static IP address is assigned ... well, statically. There is no request. Both the computer, and the ISP, are simply configured manually to "know" the IP address, and the computer that it refers to:

There's no request or response involved. (I'll now leave static IP assignments behind. Most of the rest of this applies equally well to static and dynamic assignment, except in the details of how the IP addresses are actually assigned. Dynamic is what most situations we're trying to clarify here involve.)

Now let's insert a router into the mix. It sits between the computer and the rest of the network. It requests an IP address of your ISP using DHCP:

Note that the router asked for, and got, an IP address like any other, that now identifies it on the internet.

Now, your computer behind the router, must also ask for an IP address:

The computer makes its request of the router this time, and receives an IP address back from the router. Note that the "192.168." address is special - you'll never see that on the internet, those are reserved for local networks, like the one we're building. In fact, if you connect a second computer to your router, you are in fact building a local area network:

Note that the two computers each have a unique IP address assigned from the router.

Now, what happens one of those computers makes a request that accesses the internet? The computers get to the internet through the router, so the first step looks like this:

The router must then pass on that request to the internet:

Notice: to the internet, it "looks like" the router is making the request. Remember, the internet cannot see your internal IP addresses, so the router translates from the internal address (192.168.1.100) to the external (1.2.3.4 in this example). When the response arrives, the same translation happens, in reverse:

The router keeps track of which of the computers on the LAN side the request belongs to, and routes the response appropriately.

One of the side effects of using a NAT router in this manner is that regardless of which local computer on your LAN the request came from, to the internet it all "looks like" it came from your router. In our example, even if there were 100 computers on our LAN, all the requests made of the internet would look like they came from "1.2.3.4". Because they did.

So if you're behind a router, let's look at your local computer's assigned IP address. I like to do this from a command prompt: click Start, Run, type "CMD" and press OK, then in the resulting Command Prompt window type ipconfig followed by enter:

C:\Documents and Settings\LeoN>ipconfig

Windows IP Configuration


Ethernet adapter Wireless Network Connection:

        Connection-specific DNS Suffix  . :
        IP Address. . . . . . . . . . . . : 192.168.1.106
        Subnet Mask . . . . . . . . . . . : 255.255.255.0
        Default Gateway . . . . . . . . . : 192.168.1.1

There you can see that this machine's IP address is 192.168.1.106. But if, on that same machine, I visit http://www.whatismyip.com, I get told a completely different number: "Your IP Is 206.124...." (truncated for privacy). That's the IP address of my router, on the internet. Regardless of what the IP address of the machine on my local network, on the internet I "look like" that one IP address.

There are other side effects as well, and I'll cover those in future articles.