Check the physical connection at the computer – Verify that the network cable is plugged in securely into the network card in the computer. Unplug and re-plug it to be sure it is plugged in tight and correctly. If this is a laptop using a PCMCIA card with a dongle, verify that the cable is connected to the dongle and that the dongle itself is connected securely to the network card. If the computer has multiple network cards, verify that the cable is plugged into the correct one.
Check for a damaged or misconfigured cable – While checking the physical connection, examine the network plug on each end of the cable as well as the jacks they plug into. Make sure there are no bent or missing pins. Don’t forget to try another cable to make sure the current one is not damaged. Excessive force such as sharp bends, knots or kinks can damage the wires inside the cable. As mentioned previously, depending on what devices are connected either a straight-through or a cross-over cable is required. A straight-through cable works for most situations though.
Check the physical connection at the other end – Verify that the cable is plugged securely into the network port/hub/modem/walljack on the other end. If plugged into a hub or switch, check whether the link light is on. If the link light is not lit, it could indicate a bad/incorrect cable or a bad port. Try a different port on the hub or switch. Another thing to check with a hub or switch setup is to verify that the connection from the hub or switch to the next hop such as the gateway device or cable/DSL modem is working by checking the link light on both ends of the connection. If connecting to an office network, make sure the cable is plugged into the correct port. For example, there may be several ports, one for data, one for voice. Verify with the IT department that the data jack has been activated and is connected to the local network.
Check the hardware for proper installation – If the network card is a PCMCIA card, verify it is inserted all the way into the correct slot. If it is a PCI card, verify it is seated properly in its PCI slot. If it is an onboard network adapter (built into the motherboard), verify that it is enabled via a jumper on the motherboard or a setting in the System BIOS. Make sure the network card is installed properly in the operating system. In Windows, check Device Manager for presence of the network adapter and verify that there are no conflicts or errors. To ensure there is no problem with the driver, download and install the latest driver from the manufacturer’s website.
Check the PC network configuration – In order to participate in a network, a network protocol has to be installed and configured properly. As mentioned above, the most common protocol is TCP/IP which is installed automatically by most operating systems. There are two ways to configure TCP/IP: Static or dynamic. Dynamic means that when the machine is connected to a network it will send a broadcast asking for configuration information. If there is a DHCP server on the network that observes the request, it will reply and send the machine the required information such as IP address, subnet mask, gateway IP address, and DNS server IP. In this case TCP/IP just needs to be set to obtain an IP address automatically (default configuration in Windows), no other configuration required. This configuration type is very convenient because it does not require any changes on the computer when the network configuration changes or when it is connected to a different network. Static on the other hand means that the user has to supply and manually enter this information into the TCP/IP configuration. Without the proper information the computer cannot participate in the network.
Scenario 1 – PC -> DSL/Cable modem: Check with the ISP to find out whether they use dynamic or static IP configuration. Most DSL and cable modem plans use dynamic mode, therefore it’s best to try DHCP first.
Scenario 2 – PC -> Gateway device: Here it depends how the gateway device is configured. Again, the typical setup is DHCP enabled for automatic configuration, so try this option first, unless you have specifically turned DHCP off when you set up the gateway device.
Scenario 3 – PC -> Hub/switch -> Gateway device -> DSL or cable modem: See steps for scenario 1 and 2.
Scenario 4 – PC -> Office network: Contact the network administrator to find out whether the network uses dynamic or static IP addressing. Once again, DHCP is the most common configuration, so try that first unless the network administrator tells you specifically to use a certain static IP configuration.
|TCP/IP configured for DHCP / dynamic IP addressing|
|TCP/IP manually configured with static IP configuration|
If TCP/IP is configured for DHCP, there is an easy way to check whether it has received proper IP configuration information. Open a command prompt by selecting Start / Run, typing cmd, and pressing Enter. Type ipconfig or ipconfig /all to get more detailed information and press Enter.
|ipconfig sample output|
|Sample ipconfig /all output|
|ipconfig /all output without network connection|
The same information can be accessed in Windows from the network connection panel by right-clicking on a connection and selecting Status. While at the command prompt, run the command ping 127.0.0.1. This is the loopback address and tells the PC to send a packet to itself. It is a simple test to ensure that the TCP/IP stack is working properly on this machine. If this fails, you may have to repair or reinstall TCP/IP. Next, try pinging the IP address of the PC itself (the one you just looked up using the ipconfig command) to make sure the network card is working properly. If this fails, remove the network card drivers and reinstall it.
|Windows XP Network Connection Details|
Check for firewall software – Find out if the machine is running any firewall software such as Zone Alarm. If misconfigured, it could prevent the computer from requesting or receiving DHCP configuration. If firewall software is running, turn it off temporarily while troubleshooting to eliminate it from the list of possible causes for the failure. If the connection works correctly as soon as you disable the firewall software and breaks again as soon as you reenable it, you found your culprit. Check its configuration and correct any firewall or program rules that may be blocking the connection.
Check connectivity to the next hop – When information is transported on a network, it is being handed from one station to the next like in a relay. If any of the stops along the way is broken or doesn’t know where to relay the information to next, the entire chain breaks down. Therefore it is very important to methodically check the chain and identify where exactly the breakdown occurs. The illustration shows the hops for this particular setup. The ping and tracert commands are two simple but effective tools to check for such breakdowns. The ping command sends a number of test packets to the IP address or host specified and displays whether it received a reply and how long it took. If there was no reply, you know that the destination could not be reached. The tracert command is similar but it also documents all stops between the computer it is run on and the destination with times between hops. It will show where there may be a long delay or a complete breakdown along the way so you know where to focus your troubleshooting.
Run the ipconfig /all command to find out the PC’s IP address and the gateway IP. Look at the gateway configuration to find out the ISP’s router IP address your gateway device points to. Now systematically ping every single hop by running ping (IP address) beginning with the one closest to your PC which is the default gateway. If you get a reply, ping the next one and so on until you have found one you can’t reach, indicating the possible point of failure.
|Successful ping request|
|Failed ping request|
Caveat: You have to take the ping results with a grain of salt. A failed ping does not necessarily mean a broken hop. It could just be a switch, router, or firewall that is configured to drop ICMP packets (which includes ping). If a traceroute to the destination goes past the device that doesn’t respond to ping, it’s probably not the point of failure you’re looking for.
Check DNS settings – If the network configuration checks out, but the problem is that you still cannot view any websites in your browser, the problem may be with the DNS settings. DNS stands for Domain Name Service which is the mechanism that translates a website address (URL) into the actual IP address of the webserver it resides on. When you type a URL into the address bar of your browser, it first makes a request to the ISP’s DNS server to find out what the IP address for that particular website is. Once it knows the IP address, it then sends the actual page request to that server. If no DNS server is available, the name lookup fails and it cannot access the website. Test this from a command prompt by typing nslookup pcnineoneone.com for example. If DNS is working properly, you will see the IP address displayed. If it fails you will see an error message. Alternatively, if you happen to know the IP address for a website, type it into the browser’s address bar instead of the URL and see if that pulls up the site correctly.
|Successful nslookup DNS query|
|Failed nslookup DNS query|
Run ipconfig /all and check for the existence of at least one DNS server IP address. If there is no DNS server listed, run ipconfig /renew. If that does not obtain the proper information, the resolution depends on the TCP/IP configuration. If configured for DHCP / dynamic IP addressing, the DNS server should have been provided by the DHCP server together with the IP address and gateway.
Scenario 1 – PC -> DSL/Cable modem: Contact the ISP for help.
Scenario 2 – PC -> Gateway device: Log into the gateway device and check if it received the proper DNS server information from the ISP, as well as whether it is configured to pass it on to the clients on the network via DHCP.
Scenario 3 – PC -> Hub/switch -> Gateway device -> DSL or cable modem: See scenario 2.
Scenario 4 – PC -> Office network: Check with the network administrator to have the DHCP server settings checked.
If configured with static IP information, check the TCP/IP settings and make sure that at least one valid DNS server IP address is entered. It is normal to have two or three DNS server addresses listed to have a backup in case a DNS server becomes unavailable.
IP address conflict – It is possible to receive an error message that there is an IP address conflict due to another host with the same IP address present on the same network. This usually happens when two PCs are manually configured with the same static IP address.
Scenario 1 – PC -> DSL/Cable modem: Check to make sure you entered the correct IP configuration as provided by the ISP. If that does not help, contact the ISP for further assistance.
Scenario 2 – PC -> Gateway device: Check all PCs on the network and verify that they all have unique IP addresses.
Scenario 3 – PC -> Hub/switch -> Gateway device -> DSL or cable modem: See scenario 2.
Scenario 4 – PC -> Office network: Contact the network administrator for help.
Tip: There is an easy way in Windows XP to “repair” a network connection. Navigate to Start / Control Panel / Network connections / Right-click the problematic connection / Repair, or double-click on the network connection icon in the system tray if present and select Support / Repair. This performs the following actions to attempt restoring the network connection: It broadcasts a DHCP request, flushes the local ARP (Address Resolution Protocol), NetBIOS, and DNS cache, and reregisters with WINS (Windows Internet Name Service) and DNS. If it encounters an error in the process it will display it on the screen when it’s done, providing a potential clue.
MAC address conflict – A MAC (media access control) address is a unique identification number that is assigned to each network adapter and is used to route network packets between nodes on a network. It uses the hexadecimal format and could look like this: 00-90-4B-B2-1A-86. ipconfig /all will list the MAC address for the network adapters in a computer. Any manufacturer of network adapters hardcodes a MAC address in each adapter, drawing from a range of addresses assigned to them. The first half of the MAC address identifies the manufacturer of the adapter, the second half uniquely identifies the adapter. It is extremely unlikely but theoretically possible that two devices on a network have an network adapter with the same MAC address. Because this scenario is so rare and requires advanced troubleshooting it is beyond the scope of this article.
Check for required authentication – On a corporate network network access may require additional authentication. e.g. a certificate, VPN, RADIUS, EAP, etc. It may also have MAC filtering, allowing only computers on the network whose MAC address is on a list of approved nodes.
Wireless network configuration – Troubleshooting a connection to a wireless network involves a few additional parameters to check against the configuration of the wireless access point. Each wireless network has a network name or SSID (Service Set Identifier). Check the properties for the wireless network configuration and verify it has the correct SSID entered. Wireless networks, when properly configured and secured, usually use encryption. Check whether the wireless network you’re connecting to uses encryption and if so, ensure you have the correct encryption key entered. Wireless networks also may be restricted to only accept connections from computers with an approved MAC address. Check whether this is the case and if so, whether the MAC address of your computer is on the access list.
|Windows Wireless Network Connection Properties|