How can I find out my IP address in Windows XP?

Every computer connected to the Internet is assigned an IP address. (This is a bit of an oversimplification -- some machines have more than one IP address, and sometimes several machines share a single IP address -- but the general rule still applies.) Despite the central role that IP addresses play on the Internet, Microsoft Windows XP doesn't make it a straightforward matter to figure out what your IP address is. This is because users don't generally need to know their IP address in order to use most Internet applications. There may be times, however, when you want to know what IP address you have been assigned.

The easiest way is to go to the website whatismyip.com.

For example, here's what I see when I go there:

For most purposes, that is "your IP address" at that given moment. If you visit a website, the webserver will record a visit from that IP address at that point in time. If a tech support person asks you for "your IP address", 90% of the time that's what they're going to be referring to. This website will of course also work for determining your IP address on a Mac or any other type of computer. However, IP addresses are usually assigned temporarily, so your IP address may change within a day or two.

So that's how you find the IP address of your machine as seen by "the rest of the world" when you're connected to the Internet. However, sometimes when your computer is on a home network or a work network, your computer is assigned an IP address "on the network", which is different from the IP address as seen by "the rest of the world". This happens when you have a router (often a wireless router) so that several computers in the same household can share an Internet connection. In that case, the way the machines' IP addresses work is as follows:

• The router, which manages the sharing of the Internet connection between multiple computers, would typically give itself the IP address 192.168.1.1. Dad's Machine might be assigned the IP address 192.168.1.100, and Mom's machine would be assigned the IP address 192.168.1.101. If Dad's Machine and Mom's Machine were transfering files directly between each other, they would "see" each other's IP addresses as 192.168.1.100 and 192.168.1.101.

  These IP addresses are unique within that network -- only one machine in that household (managed by that wireless router) can have the IP address 192.168.1.100 at any given time. (Although again, these are only temporary assignments, and the machine might be assigned a new IP address next time it reboots.) However, these IP addresses are not unique across the rest of the world -- if you were to go to a friend's house, their computer might be assigned the IP address 192.168.1.100 as well. There's no conflict, since your home network and your friend's home network are not directly connected to each other.

• The router, meanwhile, is connected directly to the rest of the Internet (through an Internet service provider like Verizon), and Verizon has assigned an IP address like 71.112.16.2 to the router. This is the IP address that "the rest of the world sees" when any machine in the household visits a website on the public Internet. So if Dad's Machine visits www.cnn.com and Mom's Machine visits www.cnn.com an hour later, the administrators of www.cnn.com will record two successive visits from the same IP address (and, ignoring other things like Web cookies, to the administrators of www.cnn.com it will look like two successive visits from the same computer). This IP address, 71.112.16.2, is globally unique -- unless someone elsewhere on the Internet is trying to use that IP address without authorization (and, most of the time, that won't work), your router is the only machine on the Internet that is assigned that IP address.

IP addresses that are assigned to your computer by a router on a home network, typically begin with 192.168. IP address assigned to your computer on a work network serving hundreds of machines or more (such as at a large company like Microsoft), typically begin with 10. Most of the other IP addresses that you will encounter (with a small number of exceptions), are public Internet addresses.

If you want to know your IP address on the local network, open a Windows command prompt, and in the command prompt type "ipconfig". Your IP address will be displayed along with some other information:

In brief:

• If you want to know your IP address "as seen by the rest of the world", go to http://www.whatismyip.com/ .
• If you want to know your IP address "on the local network", use ipconfig.

Bennett Haselton is a technology and political blogger who will talk your ear off about how to bypass LightSpeed, 8e6, and SmartFilter web filtering software if you let him.