Troubleshooting network problems

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By Brien Posey, MCSE

Figure A

Published in TechRepublic's Windows Support Professional

Troubleshooting network problems can be a frustrating experience, whether you're dealing with one PC or the entire ball of wax. This is because today's networks are so complicated that the point of failure could be virtually anywhere. Worse yet, your network could contain multiple points of failure, resulting in confusing symptoms that are hard to diagnose. In this article, we'll tell you how to troubleshoot networks by looking at the symptoms of the problem—for example, we'll explain how to look for a potential PC protocol mismatch, using the dialog box shown in Figure A.


Figure A To communicate, PCs must share a common protocol.

Where can a network fail?

When you're trying to troubleshoot something as complex as a network, it's often best to take what you know about the problem and gradually narrow down the possible causes. With this in mind, we'll begin with simple problems and work toward more complicated ones. Basically, only four things can cause a network to fail:

  • A problem with the server

  • A problem with a hub, router, bridge, and so on

  • A problem with the cabling

  • A problem with a workstation

Once you figure out which of these four items is causing your problem, you can begin narrowing your focus toward subcomponents.

For example, suppose you've determined that you have a cable problem. You've eliminated three quarters of your network components from the list of possible failures. Next, you'll want to investigate if you have a break in the cable, if an end is loose, if you have a missing terminator, or if you have some other problem. Let's examine the steps you'll follow in the troubleshooting process.

A starting point

The very first thing you need to find out is which PCs are affected. Are you having problems with one PC, several PCs, or every computer on the network?

Problems with a single PC

If a problem is occurring only with a single PC, you've just narrowed the cause of your problem considerably. The next question you should ask is, "Has this PC ever worked before at this location?" If it has worked before and you know that nothing has changed in the configuration, then a component may be unplugged, a cable may be broken, or a network card may have gone out.

On the other hand, if the PC is brand-new, chances are good that it's experiencing a configuration error. In a situation like this, your next step should be to plug a known good laptop computer into the network connection currently used by the failing PC. If the laptop connects to the network successfully, you know there are no cable breaks or problems with the hubs or routersthe problem is with the PC itself. If the laptop also fails to connect, you've obviously got a cable problem or a problem with a switching device (such as a hub or router).

Problems with several PCs

If you're having problems with several PCs, you need to consider which PCs are malfunctioning. If they're all new, you've probably got a configuration problem. On the other hand, if all the PCs were working previously, you most likely have a problem with a switching device or a cable segment.

At this point, you should determine what the failing PCs have in common with one another but not with the rest of the network. Many times, when multiple PCs fail simultaneously, a hub has gone bad or a cable connecting one hub to another hub has gone bad. You should check whether all the PCs share a common hub or are attached to the same cable segment.

Problems with the entire network

If no PCs on your network can log on, you've probably got problems with the server functioning as your primary domain controller. You should go to this server to begin the troubleshooting process.

Troubleshooting a PC

Because of the complexity of PCs and our space limitations, it's impossible for us to provide a comprehensive guide to troubleshooting a PC. However, we can show you how to work through some of the more common network problems.

Protocol mismatches

As you may know, in order for PCs on a network to communicate, they must share a common protocol. A protocol is a language that the computers on a network use to speak to one another. If you notice that a workstation can see only some of the other PCs on your network, there's a chance you may have a protocol mismatch.

To solve this problem, begin by double-clicking the Network icon in Control Panel. When you see the Network Properties dialog box, select the Protocols tab, as shown in Figure A. Then, compare the protocols listed for this PC with the protocols installed on a PC it can't communicate with. If the protocols match, the PCs should be able to communicate with each other.

The NetBEUI protocol is an exception to this rule. NetBEUI isn't a routable protocol. If a router is located between two PCs using NetBEUI and you haven't made special provisions, the PCs won't be able to communicate. The easiest solution to this problem is to install NWLink IPX/SPX or TCP/IP.

A word about TCP/IP

With the increasing popularity of the Internet, TCP/IP is being used more often. Because of this growing usage, we'll show you how to troubleshoot some common TCP/IP problems.

If you suspect that you have a problem with TCP/IP, the first thing you should do is PING your own IP address. If the PING returns, then TCP/IP is functional. However, this means only that the protocol is functionalit doesn't necessarily mean that TCP/IP can communicate with the rest of the world.

If the malfunctioning PC can successfully PING itself, you should PING the address of another PC on the same network segment as the malfunctioning PC. Doing so will test your network card. If the PING fails, the network card is malfunctioning.

If the PING is successful, PING the same PC using the PC's computer name rather than its IP address. If this PING fails, you probably have a problem with your WINS or DNS configuration, or possibly with your LMHosts file (if you use one).

If you can successfully PING a PC by name, it's time to PING a PC that's either on a different network segment or in the outside world. For example, you might PING If this PING fails but previous PINGs have been successful, you probably have either an incorrectly configured default gateway or a problem with your router. However, if other PCs on the segment are functioning correctly, the problem is probably the default gateway rather than the router.

Testing a network card

If you suspect that you have a bad network card, you'll need to check the card's configuration. To do so, open Control Panel and click the Network icon. When the Network Properties dialog box opens, select the Adapters tab and double-click your network adapter. When you do, you'll see any configurable parameters, as shown in Figure B.


Figure B Compare Windows NT's network card settings to the settings used by the physical card.

Many plug-and-play network cards come with special software that you can use to manually configure the card. For example, SMC cards use a piece of software called EZStart to configure such things as the card's IRQ and base memory address, as shown in Figure C. If your network card uses similar software, you should set the card's parameters to match the settings within Windows NT.


Figure C Some network cards use software for configuration.

If your network card doesn't use configuration software, it may have jumpers on the card that control these settings. If your card uses jumpers, consult the manual that came with the card to learn how to use the jumpers to make the card's settings match the settings configured through Windows NT.

Troubleshooting a cable problem

Although many types of cables are used for networking, the most commonly used fall into two categories: coax (which looks similar to the wire used for cable television) and twisted pair (which is similar to telephone cable). Because these types of cable are so dissimilar, they require different troubleshooting methods.

Troubleshooting coax

The sidebar "Coax Basics" provides some background on coax-based networks. Needless to say, most cable problems on coax-based networks affect multiple PCs. If you have a communication failure but your terminators are connected and are the correct type, you should check for a cable break. A break in the cable causes the wire to function as two separate unterminated networks, because the point at which the cable ends (the break) is unterminated.

Breaks or shorts in coax cables are often hard to find, because they aren't always visible to the naked eye. A break could be caused by something as simple as a loose T-connector. Although the wire may not be completely pulled out of the connector, it may be loose enough that it can't make a good connection. It's also tricky to locate breaks and shorts when a coax cable snakes its way through walls and conduit, under desks (where users often stack boxes on top of the wire), and into other inaccessible places.

The easiest way to troubleshoot a coax segment is to take two known good terminators and use one to terminate the cable at the source. The source is the place at which the cable connects to the server, another segment, a hub, and so on. Go to the first PC on the segment and disconnect the T-connector from the PC. Remove the portion of the line that goes to the rest of the network and replace it with the second terminator. Now, reconnect the T-connector to the PC and try to log on to the network with the PC. If the PC fails to connect, your problem is somewhere between the two terminators.

If the PC does connect, remove the terminator from the T-connector on the PC, reattach the cable to the T-connector, and repeat the process at the second PC on the line. As you get further down the line, you'll reconnect and test one PC at a time until you come to the source of the problem. If you fix a problem but the line still malfunctions, keep in mind that you may have multiple breaks on the line.

Occasionally, you may trace a problem to a particular PC and yet be unable to find a break in the line. If this happens, try disconnecting the T-connector from that PCbut enable the rest of the PCs on the line and test the network. Sometimes, a network card will go bad and flood the line with high-volume random packets. If this happens, it can cause symptoms similar to a cable break or a missing terminator.

Troubleshooting twisted pair

Twisted pair is considerably easier to troubleshoot than coax. Because of the nature of twisted pair, each line services only one PC (unless that line happens to run between two switching devices). If you suspect a cable problem in a network that uses twisted pair, the first thing you should check is the link light. If the link light isn't lit, it means that you don't have a complete physical link. Usually this indicates a break in the cable or a loose RJ-45 connector. Note that the link light can sometimes be illuminated even if you have a cable problema link light merely indicates that the wire is connected at both ends.

If the link light is lit but you still suspect a cable problem, you should plug a known good laptop computer into the cable. If the laptop establishes a network connection, the problem is with your PC. Otherwise, you have a cable problem.

Troubleshooting switching devices

It's difficult to discuss techniques for troubleshooting switching devices in an article such as this, because there are so many types of devices and our space is limited. However, you can use some general techniques to troubleshoot such a failure.

Switching device failures usually affect groups of PCs, although it's possible for only one PC to be affected. For example, a single port on a hub can go bad. If you've tried unsuccessfully to troubleshoot a cable problem and that cable is plugged into a hub on one end, try plugging it into a different port or into a different hub.

If a group of PCs goes down simultaneously, there's a good chance that the problem may be due to a failed switching device. For example, suppose you have three hubs daisy-chained together, as shown in Figure D. Now, suppose that Hub 1 controls PCs 1 through 8, Hub 2 controls PCs 9 through 16, and Hub 3 controls PCs 17 through 24.


Figure D Failures among switching devices often affect groups of computers.

Obviously, if Hub 3 fails, PCs 17 through 24 will malfunction. If Hub 2 fails, it will probably cause PCs 9 through 16 to malfunction, because they're directly connected to the hubbut it may also cause PCs 17 through 24 to malfunction, because data has to pass through Hub 2 to get to Hub 3.

If a group of PCs malfunctions simultaneously, you should compile a list of the switching devices they have in common with one another but not with the rest of the network. Although we've used hubs in our example, this principle holds true with bridges, routers, and gateways as well.

Troubleshooting server problems

Many people feel intimidated when it comes to troubleshooting a server. However, you should keep in mind that Windows NT Server and Windows NT Workstation function identically. If you're capable of fixing a workstation, you're equally capable of fixing a server (assuming your server isn't running any of the BackOffice components, such as Exchange or SQL). If your server is having trouble accessing the rest of the network, you should apply the same troubleshooting techniques we've discussed for troubleshooting a workstation.


In this article, we've discussed several common network problems. We've also provided you with methods you can use to work through these problems.

Brien Posey is an MCSE and a freelance technical writer. He also works as a network engineer for the Department of Defense. You can contact him via E-mail at

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