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The three most common causes of basic link problems are a bad cable, bad hardware or duplex mismatches. A basic link problem will show up as the link lights not lighting on one or both networking devices.
This is probably the most common reason for failure. The first thing to do when one or both of the link lights are out is to change the cable to a known working cable.
Use a standard patch cable to connect a Hub/Switch/Router to a computer. Use a crossover cable to connect two networking devices or two computers together. Some ports are able to automatically configure themselves allowing a Patch cable to be used everywhere. Be careful of homemade cables. EIA/TIA 568 cabling use 4 twisted pair. Improper termination may work at low speed but fail at high.
To learn how to correctly make an internet cable look here
B) Duplex Mismatch
There are instances where loss of link could be caused by a duplex mismatch , so it is best to have the network interface cards configured to autonegotiate for the testing. Consult the documentation on your NIC (Network Interface Card) on how to change its speed and duplex settings. Since most home switches are unmanaged there is no way to set their link speed and duplex.
C) Failed equipment
If you still do not see a link light on both devices it's time to start swapping with known working devices.
2) Basic Network connectivity.
Basic network connectivity is limited in this section to seeing other computers on your lan by using the ping tool with a network address. The various methods of name resolution are beyond the scope of this FAQ.
The most frequent causes for loss of basic network connectivity are incorrectly configured IP addresses, firewalls and a corrupted TCP/IP stack.
For a basic introduction into TCP/IP addressing click this link.
A) Correct Address scheme
To see the IP configuration on a computer with NT/2000/XP/98 open a command prompt and type in ipconfig /all. The GUI command in Windows 9X is winipcfg. If one of the two machines is a router see its documentation on how to find the LAN address.
Verify that both machines are on the same subnet. If the addresses are statically assigned then correct one of the machines. Most home lans use automatically assigned (DHCP) addressing with the address coming from a router or an Internet connection sharing (ICS) server.
When one of the devices is set up to hand out the IP address verify that the second device is configured to receive an ip address automatically. This link explains setting up the adapter for 2K/XP, follow these instructions for 98/NT.
If you get an address in the 169.254.x.x range after configuring for basic link then the DHCP server is not working or the client is unable to see the DHCP server. Also ensure that the DHCP server's scope is large enough for the amount of PC's on the lan.
If the address is 0.0.0.0 then the problems is most likely the TCP/IP stack.
B) Firewalls blocking
Assuming you have two devices 192.168.0.2 & 192.168.0.3 open a command prompt on .2 and type ping 192.168.0.3. From 192.168.0.3 open a command prompt and type ping 192.168.0.2. If you get replies back both times you're done, good job.
If you received "Request timed out" errors then a firewall on the receiving machine (the one you're not on) is preventing it from replying. Consult the firewall documentation or help for instructions on setting up a "trusted zone". Windows XP has a built in firewall that is turned on as part of the Network Wizard this link explains how to enable and disable ICF.
Having XP's ICF and another firewall on at the same time can cause problems, so pick one firewall and stick with it. If the "trusted zone" or lan is set up and the pings still time out the firewalls must be uninstalled until the network is working. Do not leave the internet connection up with no firewalls installed.
Note: Ping uses Internet Control Message Protocol (ICMP). The Firewall may be configured to pass Ping while blocking ports used for TCP and UDP. If Ping works but other access does not suspect a misconfigured Firewall.
C)Corrupt TCP/IP stack
If all of the above steps are followed or you received a ping reply other than request timed out then the problem is in the TCP/IP stack. This link explains troubleshooting the stack. If the stack was corrupted because of malware, the malware must be completely removed before the stack can be repaired.
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