Basic network troubleshooting

If a computer is unable to connect to a network or see other computers on a network, it may be necessary to troubleshoot the network. A network may not work because of any of the below reasons.
1. Network card not connected properly.
2. Bad network card drivers or software settings.
3. Firewall preventing computers from seeing each other.
4. Connection related issues.
5. Bad network hardware.

Solution:
Because of the large variety of network configurations, operating systems, setup, etc... not all of the below information may apply to your network or operating system. If your computer is connected to a company or large network, or you are not the administrator of the network, it is recommended that if you are unable to resolve your issues after following the below recommendations that you contact the network administrator or company representative.
Note: If you are being prompted for a Network password and do not know the password, Computer Hope is unable to assist users with obtaining a new or finding out the old password.
Verify connections / LEDs
Verify that the network cable is properly connected to the back of the computer. In addition, when checking the connection of the network cable, ensure that the LEDs on the network are properly illuminated. For example, a network card with a solid green LED or light usually indicates that the card is either connected or receiving a signal. Note: generally, when the green light is flashing, this is an indication of data being sent or received.
If, however, the card does not have any lights or has orange or red lights, it is possible that either the card is bad, the card is not connected properly, or that the card is not receiving a signal from the network.
If you are on a small or local network and have the capability of checking a hub or switch, verify that the cables are properly connected and that the hub or switch has power.
Adapter resources
Ensure that if this is a new network card being installed into the computer that the card's resources are properly set and/or are not conflicting with any hardware in the computer.
Users who are using Windows 95, 98, ME, 2000 or XP, verify that Device Manager has no conflicts or errors. Additional help and information about Device Manager and resources can be found on our Device Manager page.
Adapter functionality
Verify that the network card is capable of pinging or seeing itself by using the ping command. Windows / MS-DOS users ping the computer from a MS-DOS prompt. Unix / Linux variant users ping the computer from the shell.
To ping the card or the localhost, type either
ping 127.0.0.1
or
ping localhost
This should show a listing of replies from the network card. If you receive an error or if the transmission failed, it is likely that either the network card is not physically installed into the computer correctly, or that the card is bad.
Protocol
Verify that the correct protocols are installed on the computer. Most networks today will utilize TCP/IP, but may also utilize or require IPX/SPX and NetBEUI.
When the TCP/IP protocol is installed, unless a DNS server or other computer assigns the IPX address, the user must specify an IP address as well as a Subnet Mask. To do this, follow the below instructions.
1. Click Start / Settings / Control Panel
2. Double-click the Network icon
3. Within the configuration tab double-click the TCP/IP protocol icon. Note: Do not click on the PPP or Dial-Up adapter, click on the network card adapter.
4. In the TCP/IP properties click the IP address tab
5. Select the option to specify an IP address
6. Enter the IP address and Subnet Mask address, an example of such an address could be:IP Address: 102.55.92.1Subnet Mask: 255.255.255.192
7. When specifying these values, the computers on the network must all have the same Subnet Mask and have a different IP Address. For example, when using the above values on one computer you would want to use an IP address of 102.55.92.2 on another computer and then specify the same Subnet Mask.
Firewall
If your computer network utilizes a firewall, ensure that all ports required are open. If possible, close the firewall software program or disconnect the computer from the firewall to ensure it is not causing the problem.
Additional time
In some cases it may take a computer some additional time to detect or see the network. If after booting the computer you are unable to see the network, give the computer 2-3 minutes to detect the network. Windows users may also want to try pressing the F5 (refresh) key when in Network Neighborhood to refresh the network connections and possibly detect the network.
Additional troubleshooting
If after following or verifying the above recommendations you are still unable to connect or see the network, attempt one or more of the below recommendations.
If you have installed or are using TCP/IP as your protocol you can attempt to ping another computer's IP address to verify if the computer is able to send and receive data. To do this, Windows or MS-DOS users must be at a prompt and Linux / Unix variant users must open or be at a shell.
Once at the prompt assuming, that the address of the computer you wish to attempt to ping is 102.55.92.2, you would type:
ping 102.55.92.2
If you receive a response back from this address (and it is a different computer), this demonstrates that the computer is communicating over the network. If you are still unable to connect or see the network, it is possible that other issues may be present.
Another method of determining network issues is to use the tracert command if you are a MS-DOS or Windows user or the traceroute command if you are a Linux / Unix variant user. To use this command you must be at the command prompt or shell.
Once at the prompt, assuming that the address is again 102.55.92.2, type:
tracert 102.55.92.2
or
traceroute 102.55.92.2
This should begin listing the hops between the computer and network devices. When the connection fails, determine which device is causing the issue by reviewing the traceroute listing.

Troubleshooting Tips for a Network Card

If you can't connect to a server or if Windows reports an error after you double-click Network Neighborhood, try the following first:

• Check whether the driver for the network interface card (NIC) is correct or not.
• Check whether the driver is loaded and the protocols are bound.
• Check the Device Properties list for trouble indicators (an "X" or "!" symbol).
  Test the NIC adapter with the diagnostic utilities that often came with the driver installation.
• Try to reinstall additional networking software.
  

If the problem persists, follow these guidelines:

• Make sure the cable is installed properly. The network cable must be securely attached at both RJ45 connections (adapter and hub). The maximum allowable distance from adapter to hub is 100 meters. If the cable is attached and the distance is within acceptable limits but the problem persists, try a different cable. If you're directly connecting two computers without a hub or switch, use a crossover cable.
• Try another network cable.
• Check the LED Lights on the NIC. Before the LEDs can be used for troubleshooting, the network interface card (NIC) must be connected to the network and the network driver must be installed. Most NICs come with LEDs near the connection. The meaning of the LED signals may be different from one manufacturer to the other. Here is a common LED description for 3COM 10/100BT dual speed NIC. Please consult your NIC manual for any difference.

The computer hangs when the drivers are loaded.

• Change the PCI BIOS interrupt settings. See your NIC and system manuals for more details.
• If you are using EMM386, it must be version 4.49 or newer.
  Diagnostics pass, but the connection fails or errors occur.
  At 100BT, use Category 5 wiring and make sure that the network cable is securely attached.
• At 100BT, connect to a 100BT hub/switch (not 100Base-T4).
  For NetWare, make sure you specify the correct frame type in your NET.CFG file.
  Make sure the duplex mode setting on the adapter matches the setting on the switch.
  

The LNK LED doesn't light.

• Make sure you've loaded the network drivers.
• Check all connections at the adapter and the hub/switch.
• Try another port on the hub/switch.
• Make sure the duplex mode setting on the adapter matches the setting on the hub/switch.
• Make sure you have the correct type of cable between the adapter and the hub. 100Base-TX requires two pairs. Some hubs require a crossover cable while others require a straight-through cable.
• The ACT LED doesn't light.
• Make sure you've loaded the correct network drivers.
• The network may be idle. Try accessing a server.
• The adapter isn't transmitting or receiving data. Try another adapter.
• Make sure you're using two-pair cable for TX wiring.
• The adapter stopped working without apparent cause.
• Run the diagnostics program that came with the NIC.
• Try reseating the NIC in its slot, or try a different slot if necessary.
• The network driver files may be corrupt or missing. Remove the drivers and then reinstall them.

The Wake on LAN (WOL) feature is not working.

• Make sure the WOL cable is attached and that power is being applied to the computer.
  Check the BIOS for its WOL setting. Some computers may need to be configured for WOL.
• Make sure the network cable is fully attached to the adapter.
  Crossover cable troubleshooting tips. When you work with network cabling, concentrators (hubs or switch), and NICs from different venders, it is possible to connect everything and still have no communication between file servers and workstations. When there are several unknown variables, it is difficult to determine which component is broken. Use these tips to isolate the problem.
• Find whether your equipment complies with the 10Base-T or 100Base-TX standard. This is particularly important for hubs and switches.
• Connect a straight-through cable from the PC to the hub. The hub performs an internal crossover so that the signal can go from TD+ to RD+ and TD- to RD- (see How to Make Network Cables). When you look at an RJ-45 connector from the front, pin 1 is identified on the left-hand side when the metal contacts are facing up.
• Make sure that the TD+ and TD- wires are twisted together , and that the RD+ and RD- wires are twisted together. Using wires from opposing pairs can cause signals to be lost. For a 100Base-TX cable, pins #1 and #2 , and #3 and #6 must be on the same twisted strand.

When there is doubt whether a hub is performing correctly, or if the impedance settings are in question, a crossover cable can help you isolate the failing component:

• Connect a file server and a client PC back to back with a crossover cable to verify that the NIC and network operating system are properly configured.
• To make a crossover cable, simple connect TD+ to RD+ and TD- to RD-. The cable performs the crossover that is usually performed by the hub. Make sure that two twisted-pair wires are used. If the file server and client PC function together as a small network, then either the existing cabling or the hub is the problem.
• If there is a proper crossover, the appropriate LED comes on. If there is a straight-through connection, the LED does not light. A blinking LED indicates that there is a polarity mismatch (that is, TD+ to RD- instead of TD+ to RD+).