How to Troubleshoot Network Connectivity Problems

If you have a network connectivity problem, you will be unable to reach any network resource that can normally be accessed using the failed network. For example, if your Internet connection has failed, you will be unable to access Internet resources, but you might still be able to access resources on your LAN. If your LAN fails, however, nothing will be accessible. Most network connectivity problems result from one of the following issues:

• Failed network adapter
• Failed network hardware
• Failed network connection
• Faulty network cables
• Misconfigured network hardware
• Misconfigured network adapter

Note Often, people jump to the conclusion that the network has failed when only a single network resource has failed. For example, a failed DNS server will stop your computer from resolving host names, which would prevent the computer from finding resources on the network by name. Similarly, if the only network resource a user accesses is her e-mail server and that server has failed, the failure might appear to that user to be a total loss of connectivity. To avoid spending time troubleshooting the wrong problem, the processes in this tutorial always start by isolating the cause of the problem.

After you isolate the failed feature, you can work to resolve that specific problem or you can escalate the problem to the correct support team. For example, if you determine that the network adapter has failed, you will need to contact the hardware manufacturer for a replacement part. If you determine that the Internet connection has failed, you will need to contact your ISP. To isolate the cause of a network connectivity problem, follow these steps:

1. Open the Network And Sharing Center by clicking the network icon in the system tray and then clicking Open Network And Sharing Center. At the bottom of the page, click Troubleshoot Problems and follow the prompts that appear. If Windows Network Diagnostics does not identify or resolve the problem, continue following these steps.
2. Open a command prompt on the computer experiencing the problems. Run the command ipconfig /all. Examine the output as follows:
   • If no network adapters are listed, the computer either lacks a network adapter or (more likely) it does not have a valid driver installed.
   • If all network adapters show a Media State of Media Disconnected, the computer is not physically connected to a network. If you are using a wireless network, see the section titled "How to Troubleshoot Wireless Networks" later in this tutorial. If you are using a wired network, disconnect and reconnect both ends of the network cable. If the problem continues, replace the network cable. Attempt to connect a different computer to the same network cable; if the new computer can connect successfully, the original computer has a failed network adapter. If neither computer can connect successfully, the problem is with the network wiring, the network switch, or the network hub. Replace the network hardware as necessary.
   • If the network adapter has an IPv4 address in the range of 169.254.0.1 through 169.254.255.254, the computer has an APIPA address. This indicates that the computer is configured to use a DHCP server, but no DHCP server is available. With administrative credentials, run the following commands at a command prompt.
     

     ipconfig /release
     ipconfig /renew
     ipconfig /all
     

     If the network adapter still has an APIPA address, the DHCP server is offline. Bring a DHCP server online and restart the computer. If the network does not use a DHCP server, configure a static or alternate IPv4 address provided by your network administration team or your ISP.
   • If all network adapters show DHCP Enabled: No in the display of the ipconfig /all command, the network adapter might be misconfigured. If DHCP is disabled, the computer has a static IPv4 address, which is an unusual configuration for client computers. Update the network adapter IPv4 configuration to Obtain An IP Address Automatically and Obtain DNS Server Address Automatically. Then configure the Alternate Configuration tab of the IP Properties dialog box with your current, static IP configuration. For information about configuring IP addresses.
     For most networks, set client configuration to Obtain An IP Address Automatically.
3. Having arrived at this step, you know that your computer has a valid, DHCP-assigned IPv4 address and can communicate on the LAN. Therefore, any connectivity problems are caused by failed or misconfigured network hardware. Although you cannot solve the problem from a client running Windows, you can still diagnose the problem. View the output from the ipconfig command and identify the IPv4 address of your default gateway. Verify that the IPv4 address of the default gateway is on the same subnet as the network adapter's IP address. If they are not on the same subnet, the default gateway address is incorrect-the default gateway must be on the same subnet as the client computer's IPv4 address.
   Note To determine whether an IPv4 address is on the same subnet as your computer's IPv4 address, first look at your subnet mask. If your subnet mask is 255.255.255.0, compare the first three sets of numbers (called octets) in the IPv4 addresses (for example, 192.168.1 or 10.25.2). If they match exactly, the two IPv4 addresses are on the same subnet. If your subnet mask is 255.255.0.0, compare the first two octets. If your subnet mask is 255.0.0.0, compare only the first octet (the first grouping of numbers before the period in the IP address). If any of the numbers in the subnet mask are between 0 and 255, you will need to use binary math and the AND operation to determine whether they are on the same subnet.
4. Attempt to ping the default gateway using the following command.
   ping default_gateway_ip_address
   For example, given the following IPConfig output:

   Ethernet adapter Local Area Connection:
   	Connection-specific DNS Suffix .  :
   	Link-local IPv6 Address . . . . . : fe80::1ccc:d0f4:3959:7d74%10
   	IPv4 Address. . . . . . . . . . . : 192.168.1.132
   	Subnet Mask . . . . . . . . . . . : 255.255.0.0
   	Default Gateway . . . . . . . . . : 192.168.1.1
   

   you would run the following command.
   ping 192.168.1.1 If the Ping results show "Request timed out," your computer has the incorrect IP address configured for your default gateway, your default gateway is offline, or your default gateway is blocking ICMP requests. If the Ping results show "Reply from ...," your default gateway is correctly configured, and the problem is occurring elsewhere on the network.
   
   Note Ping is not a reliable tool for determining whether computers or network equipment are available on the network. Today, to reduce security risks, many administrators configure devices not to respond to Ping requests. However, Ping is still the most reliable tool for testing routers, and most administrators configure routers to respond to Ping requests from the local network. It's a good idea to ping your network equipment when everything is working properly just to determine whether it responds under normal conditions.
5. Use the Tracert command to test whether you can communicate with devices outside your LAN. You can reference any server on a remote network; however, this example uses the host www.yahoo.com.
   C:\>tracert www.yahoo.com
   The 0 line is your client computer. The 1 line is the default gateway. Lines 2 and above are routers outside your local area network.
   • If you see the message "Unable to resolve target system name," your DNS server is unreachable because the DNS server is offline, your client computer is misconfigured, or the network has failed. If your DNS server is on your LAN (as displayed by the ipconfig /all command) and you can still ping your router, the DNS server has failed or is misconfigured; see the section titled "How to Troubleshoot Name Resolution Problems" later in this tutorial for more information on these issues. If your DNS server is on a different network, the problem could be either a network infrastructure problem or a name resolution problem. Repeat this step, but use Ping to contact your DNS server IP address (as displayed by the ipconfig /all command). Then, follow the steps outlined in the section titled "How to Troubleshoot Name Resolution Problems" later in this tutorial to further isolate the issue.
   • If nothing responds after line 1, your default gateway cannot communicate with external networks. Try restarting the default gateway. If the default gateway is connected directly to the Internet, the Internet connection or the device that connects you to the Internet (such as a cable or DSL modem) might have failed. Contact your ISP for additional troubleshooting.
   • If the same gateway appears multiple times in the Tracert route, the network is experiencing a routing loop. Routing loops can cause performance problems or cause communications to fail entirely. Networks typically fix routing loops automatically; however, you should contact your network support team to make sure they are aware of the problem. The following Tracert output demonstrates a routing loop, because nodes 5, 6, and 7 repeat.
     C:\>tracert www.yahoo.com
   • If any routers on line 2 or above respond (it doesn't matter if the final host responds), the client computer and the default gateway are configured correctly. The problem exists with the network infrastructure, or your Internet connection may have failed. Follow the troubleshooting steps described in the next section, "How to Troubleshoot Application Connectivity Problems," or contact network support to troubleshoot the problem.

To double-check your results, repeat these steps from another client computer on the same network. If the second client computer exhibits the same symptoms, you can be confident that part of the network infrastructure has failed. If the second client can successfully communicate on the network, compare the IPConfig /all output from the two computers. If the Default Gateway or DNS Server addresses differ, try configuring the problematic computer with the other computer's settings. If this does not resolve the problem, the problem is unique to the problematic computer and may indicate a hardware or driver problem.