Begin network troubleshooting by determining if there has been any recent change to the system. If there has been a change to the system, it might be causing the problem. Reverse the changes and see if the problem is resolved. If so, then the problem was with the installation.

If there have been no changes, the next area to troubleshoot is the physical layer. Start outside the computer case with the cabling. Verify that the cabling is attached to the back of the computer. The cabling should run from the wall jack to the back of the computer or external modem. Verify that the cable is attached to the correct wall jack or port and that the cable is "good". The NIC and modem provide basic troubleshooting information by using onboard lights. Most NIC cards have a green blinking light that signifies data is being processed. This does not mean the computer has an established network connection. This blinking green light tells the technician that the NIC is recognized and properly set in its slot. Figure shows the lights on a NIC.

Usually, modems have indicator lights that can be used to diagnose a problem. Each modem manufacturer has a different set of lights, but generally there will be power, status, and activity lights. This light can help isolate the problem. For example, the status light can indicate the modem is not online. The technician could then refresh the IP address to get the modem back online.

If the problem still exists after all the connections have been verified, move inside the computer box. Check to make sure the NIC card or modem is properly inserted into the expansion slot. Technicians can remove and reinstall if they feel the card has been improperly installed.

Most network adapter problems will involve conflicting resources and configuration settings. Common resource problems are conflicting IRQs, I/O addresses, and memory. Common configuration errors include adapter card speed mismatch and bad drivers.