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When you start up Firefox or Internet Explorer, the last thing you want to see is a plain, white screen declaring 'The page cannot be displayed'. But if you ever do encounter one, we have the fix. Whether you work in Windows XP or Vista, tracking down connection failures can be a simple, methodical process.

Both XP and Vista are supposed to be advanced, graphical operating systems, but when your connection goes down, you'll find the solution in the command prompt--specifically, the Ipconfig command. Short for "Internet Protocol configuration," Ipconfig is key to establishing and reestablishing your Internet connectivity. Invariably, this command, with its array of appended parameters, is what your ISP's tech support rep will tell you to try when you're having difficulty getting online.

Here are five ways that you can raise your IP IQ and troubleshoot your connection yourself.

Web Connection Woes? Refresh Your IP Address

If you use DSL or cable broadband, you probably have a dynamic IP (Internet Protocol) address, which means that your PC's In­ ternet address changes each time you log on. Your IP address should be assigned by DHCP (Dynamic Host Control Protocol).

From time to time, however, this process doesn't be­ have as dynamically as its name implies: Sometimes it doesn't assign a new address at boot-up, and you're stuck with the previous address. Since a fresh IP address is necessary to reach the Internet, when you open your browser, all you see is an error screen stating that the PC cannot establish a connection to the Internet.

Whether your system connects directly through a modem or via a router, the first step in getting an IP address assigned is to right-click the network icon in the system tray. From the resulting menu, select Repair. Windows will automatically flush the old ad­ dresses and request new ones from your router or Internet service provider, depending on how your PC is connected.

Most of the time, this op­ eration works like a charm. But when it doesn't, you'll have to troubleshoot the situation manually, and this is where knowing the ins and outs of Ipconfig can help you get your connection up and running.

Click Start, Run and type cmd. In Vista you can save a step simply by typing cmd in the Start Search box.

At the command prompt, type ipconfig to see your currently known IP address, the subnet mask, and the default gateway for all adapters. Other adapters might in­ clude Wi-Fi and Bluetooth cards, although they may be listed as disconnected.

By itself, Ipconfig does nothing more than display information. (Click the image here for a detailed view of Ipconfig's output.) To make it actually do something--like refreshing your IP address--you must add parameters preceded by a space and a forward slash. The two parameters that do the most effective job of repairing your Internet connection are '/release' and '/renew'.

Typing ipconfig /release in­ structs the DHCP server to erase the existing IP address for all adapters, be they ethernet or wireless. The process should take a few seconds, confirmed with a display in the DOS box showing all zeros for the IP address and subnet mask.

Now type ipconfig /renew. If the command is successful, a new IP address, a subnet mask, and the default gateway will appear along with the DNS suffix (basically your ISP's address, such as comcast.net).