Network connectivity issues can easily be caused by physical network problems. Always ensure that you have a reliable and working connection between each computer and another, or each computer and a router.
Diagnosing a possible problem with a wired connection requires checking 3 components.
- The network card on this computer.
- The network cable.
- The network card on the other computer, or the router port.
Does your computer connect to another computer or to a router? If to a router, try another port on the router - preferably swap ports with another, working computer. Also, swap network cables with another computer. Always test with currently known good components, when possible. And please, always start with a pre-made network cable - this is NOT the time to try making your own Ethernet cable!
Remember that the network card on this computer, and the network card on the other computer (or the router) are all computers in their own right. Each device uses drivers and / or firmware. Check with the vendor, and see if this is a known problem, and / or is there a driver or firmware update available? Whenever you have a problem, start with updated software. If you ever go the vendor for advice, that is the first thing they will ask you about.
Examine the network card, or read the manual. Make sure that it doesn't contain an embedded hardware firewall, like the nVidia nForce.
Also, look at whether the problem is constant and permanent, or chronic. If chronic, is there a pattern to when it occurs, and / or is there a consistent workaround? One common example of this possibility would be loss of connectivity when the computer is idle for an hour or so.
Read Practically Networked Problems with Network Cards for suggestions on dealing with problems that originate with a network card itself.
Run the Device Manager (System Properties - Hardware tab), find the network adapter in question, and Troubleshoot it. See if the system can identify a hardware problem.
Always use the right kind of network cable, and always have a spare on hand. If you are connecting a computer to a router, you'll probably use a straight-thru aka patch cable. If you are connecting two computers directly, you will probably need a cross-over aka null-modem cable. Some newer network cards may support use of a straight-thru cable, to connect directly to another computer. But always have a spare cross-over cable to diagnose this problem.
Does the network card, and maybe the router port, have one or two colored lights that light up or change color? Observing their behaviour, and checking the owners manual, could save you a trip to your nearby computer store to buy the wrong component.
On most network cards, the Green light indicates Link (connectivity), while the Yellow light indicates Transmission (activity). The Green light should be solid, while the Yellow light may be either blinking (light activity), or solid (heavy activity).
Windows XP has the Local Area Connection status indicator in the tooltray. Windows Vista has the Network System Icon. Both will indicate logical network status, in some detail, if enabled (Local Area Connection Properties "Show icon ...").
If a router is the other end of the connection, try checking the router access log too. Having a second computer is very useful here.
If your network includes WiFi components, your issues may be even more complex to diagnose.
- WiFi operates at Layer 2 of the OSI Network Model, but it has its own network setup procedures.
- Besides networking, WiFi involves radio (really). The WiFi radio signal won't be uniformly available in your work area; however, it will leak into your neighbours work area (and your neighbours signal into your area).
- You can control your own network, but you cannot control your neighbours network. And your neighbours network use will affect your network.
- You must protect your network from your neighbour's use of it, intentional or not.
... or maybe not. If you have no connectivity at all, check that the radios on the computer and on the router / AP are powered on. Seriously.
For more information about WiFi, see Microsoft: A Support Guide for Wireless Diagnostics and Troubleshooting.
And in some cases, if you have XP SP2 on the computer, the (KB893357): WPA2 / WPS IE update for XP SP2 may help.
For truly unacceptable problems, prepare to uninstall the drivers for one or more network adapters.