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 Internet 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 behave 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 addresses and request new ones from your router or Internet service provider, depending on how your PC is connected.

Most of the time, this operation 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 include Wi-Fi and Bluetooth cards, although they may be listed as disconnected.

Ipconfig basic information

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 instructs 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).

The ABCs of DNS

Sometimes you might have no trouble getting an Internet connection, yet for some mysterious reason you can't connect to a particular site that you easily reached previously. The problem most likely lies in the DNS client resolver cache. The Domain Name System (DNS) translates Internet-domain text names into IP numbers. For example, Amazon.com becomes 207.171.171.132.

The cache keeps a record of the sites you've recently visited and speeds up return visits to that site later. Occasionally part of this file can get corrupted, making it hard for your PC to remember where to find the sites you want. Cleaning it up usually resolves your site connection issues--and Ipconfig does the trick here, too.

First type ipconfig /displaydns, being sure to leave a space between the 'ipconfig' command and the slash. This will display a record of frequently visited or recently visited sites. See if the problematic site is listed. Even if it isn't, next type ipconfig /flushdns, which does just what you'd expect and removes all listings. Now try accessing the troublesome site. You should be back in business.

Get the 411 on Ipconfig

Ipconfig commands

Ipconfig has many more seemingly arcane parameters for advanced levels of troubleshooting, which, if you're fortunate, you'll never need. To bring up a quick listing of all Ipconfig parameters with skeletal descriptions of what each does, type ipconfig /? or ipconfig /help (again, be sure to leave a space after the 'ipconfig' command).

The list of available parameters for Ipconfig is roughly twice the height of the window, so be sure to scroll up to see the first half or drag the window border downward to view the entire list at once.

To close the DOS window after you've finished entering commands, type exit at the command prompt. You can also click the X at the top right of the window.

The Ping Thing

Meanwhile, back at the DOS command prompt, another essential network troubleshooting tool is the Ping command, a great utility for verifying your PC's network connectivity. Pinging sends data packets of a specific size to a host (a Web address or any IP address). Just as submarines use sonic pings to measure distances to underwater objects, this command estimates the round-trip time to echo the packets back, and lists any packet loss.

Ping activity

A smart first step is to ping your computer's local loopback address (127.0.0.1). At the command prompt, type ping 127.0.0.1 and wait a few seconds for a response. Windows will attempt to ping your system's network card, to see if it's working. If you receive packets back, you know your network adapter is okay. Now ping a specific external address by typing, for example, ping google.com . (Click the image here to see a detailed view of typical ping activity.) If packets return, your Net connectivity is established, and you should be able to surf once more.

If, however, you can't get a response from an external site, try pinging the IP address of your default gateway. (You can obtain this address by using the Ipconfig command.) If you ping the default gateway and don't receive a response, it means Windows can't communicate with your router, your modem, or both. And in that case, you need to do a hardware check of your modem and/or router, and of the cables connected to them.

A Physical Checkup

If software troubleshooting commands don't restore your Internet connection, the problem may be physical. If your PC is connected directly to your DSL or cable modem, make sure that all of the cables are attached tightly and all of the modem lights are lit. Also look at the ethernet port on your PC to verify that the link light is on; this will be a solid or blinking light positioned where the ethernet cable from the modem connects to the PC.

If the link light is off but the ethernet cable is plugged into a powered-on PC, that's a clear indication that you need to replace the ethernet cable.

If the link light is on and you don't have connectivity, power off all of your network devices, wait a minute, and then power them back on. It is important to do these steps in the correct order:

  1. First turn off your PC, then unplug the power cord from your modem. If you are using a router, remove its power cord as well.
  2. Restart the modem next. After it has reset and its lights indicate online connectivity, power up your router and wait for it to reset.
  3. Once that is done, turn on your PC, open your browser, and check to see if you are connected.

If all of that fails, you've done what you could, and it's time to call your ISP's tech support. Explain to the representative what you have previously tried so you don't have to do it all over again (although some support technicians will insist on starting from square one). The rep can then proceed to apply more-advanced tech muscle to your problem, or possibly track the trouble to something on the provider's end of the line.