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Troubleshooting a Network Connection

Modern computers seem almost useless these days without network connectivity.  So when your computer can't communicate with other computers or devices on your local home or business network it can be frustrating.  Its even more difficult when you can't access important web sites or being able to communicate with remote family, friends and colleagues.

Below are some steps to help you troubleshoot problems with your computer connection to your local network and the Internet.  

A basic home/small business network looks like this:
Computer <-> Network Cable <-> Router/Wireless Switch <-> Modem (DSL/Cable/Fiber) <-> ISP/Internet

A basic corporate network is a lot more complex looks like this:
Computer <-> Network Cable <-> Switch <-> Router <-> (Network Services) <-> Firewall <-> ISP/Internet
Note: The "network service" part of this model can be more ambiguous, because it can contains services Active Directory (or some other type of directory service), NAP (Network Access Protection), IPSEC, VLANs, etc...

One of the first things that you should do is determine if you're using a dynamic or statically assigned IP address.  Most computer on a corporate network or home network generally use dynamic IP addresses which is automatically assigned to them as soon you connect to the network.  While most servers in an organization is going to generally be using a static IP address that is permanently assigned to the device.

Computers with a Static IP address:
  • Make sure the static IP address is still correct, check with the network administrator if there has as been any subnet changes to the network.  If you type IPCONFIG /ALL and the IP address is in the 169.254.XXX.XXX range (known as a "Private Network"), this can be an indication that you not connected to the network.  Another indicator that you're computer is using a static IP address is the "DHCP Enabled" field in output below will be "No".

I have included a sample of the output from an IPCONFIG /ALL command below, the critical areas are highlighted:

C:\Windows\system32>ipconfig /all

Windows IP Configuration

   Host Name . . . . . . . . . . . . : XXXXXXXXXXXXXXXXX
   Primary Dns Suffix  . . . . . . . :
   Node Type . . . . . . . . . . . . : XXXXXXXXXX
   IP Routing Enabled. . . . . . . . : XXXX
   WINS Proxy Enabled. . . . . . . . : XXXX
   DNS Suffix Search List. . . . . . : XXXXXXXXXXXXXXXXX

Ethernet adapter Local Area Connection:

   Connection-specific DNS Suffix  . : XXXXXXXXXXXXXXXXX
   Description . . . . . . . . . . . : XXXXXXXXXXXXXXXXX
   Physical Address. . . . . . . . . : XXXXXXXXXXXXXXXXX
   DHCP Enabled. . . . . . . . . . . : XXXX
   Autoconfiguration Enabled . . . . : XXXX
   Link-local IPv6 Address . . . . . : XXXXXXXXXXXXXXXXX
   IPv4 Address. . . . . . . . . . . : XXX.XXX.XXX.XXX
   Subnet Mask . . . . . . . . . . . : XXX.XXX.XXX.XXX

   Lease Obtained. . . . . . . . . . : XXXXXXXXXXXXXXXXX
   Lease Expires . . . . . . . . . . : XXXXXXXXXXXXXXXXX
   Default Gateway . . . . . . . . . : XXX.XXX.XXX.XXX
   DHCP Server . . . . . . . . . . . : XXX.XXX.XXX.XXX
   DHCPv6 IAID . . . . . . . . . . . : XXXXXXXXXX
   DHCPv6 Client DUID. . . . . . . . : XXXXXXXXXXXXXXXXX

   DNS Servers . . . . . . . . . . . : XXXXXXXXXXXXXXXXX
                                       XXXXXXXXXXXXXXXXX

   NetBIOS over Tcpip. . . . . . . . : XXXXXXXXXX

Computers with a DHCP IP address:
  • Check to make sure that computer is picking up DHCP address from the server.  If you type IPCONFIG /ALL and the address range is 169.254.XXX.XXX range, its generally a good indication that you not connected to the network or your computer can't communicate with the DHCP server. One indication that you're computer is using a dynamic IP is the "DHCP Enabled" in output below will be "Yes".
Try following commands from an administrator prompt, this will renew your DHCP address:
ipconfig /release
ipconfig /renew

Things to Check
Below is a basic list things that you can check to help you diagnose the problem:
  • Find out if other people on the network are having the same problem as you.  This will help you to isolate if its just your computer or everyone's computer.
  • Ping the localhost, default gateway, and another remote machine.  Use the following command
    • ping localhost or 127.0.0.1, if this doesn't work there is a low-level problem with your network (e.g. driver or hardware)
    • ping <default_gateway>, this address will vary depending on the network you're on.  Look at the output from the IPCONFIG command to find the default gateway.  If you can make to the default gateway, but you can't get passed it then there might be a problem with your router/switch/ISP.  If you're on a basic home/small business network, log into your switch and see if its connected to Internet.
    • ping <website (e.g. google.com)>, If you can get to your default gate, but can't see the Internet their could be a your router's configuration or hardware, or possibly the ISP's Modem or connection.
  • Check the adapter status (e.g. is it able to see network cable, and/or network), and see what its display (in the Control Panel open the Network and Sharing Center, and click the link for Change Adapter Settings).  Try resetting the adapter (right-click the adapter and select Diagnose) if necessary.  Also make sure that there are no teaming issues if the adapters are teamed.
Note: If this is a small home/business network and you can't see the Internet:
  • Make sure your router/modem can see the Internet and there are no problems with your ISP. 
  • First things to try is resetting/re-seating all your cable connect connections from the computer to the router to the modem. 
  • Then restart your modem first then your router.  If that doesn't work call your ISP to make sure that there are no problems.
  • Check the Event Logs and see if there were any errors (e.g. duplicate IP addresses, etc.).
  • Check the Device Manager make sure that Windows can talk to the device drivers. 
  • Try restarting your computer and testing the connection again.
  • Call your ISP and find out if there is a problem with your network connectivity.
  • Try updating the OS, driver and the firmware of your network (if possible)
  • Try plugging your computer directly into the modem and see that fixes the problem.
    • If bypassing the switch and going directly into the modem fixes the problem then your switch/WAP could be bad.
    • If bypassing the switch and going directly into the modem doesn't fix the problem then your NIC, cable, or modem could be bad.

If you're in a corporate environment:
  • Check If there is any type of change control, find out if there was any driver update that happen recently or any changes to the OS or network.  You can also check the system up-time to see if it was rebooted recently this might give hint to software update.  If the system has been rebooted, it might be worthwhile to try to reboot to see if the network adapter comes back online.
  • Make sure that there are no issues Local Firewall, IPSEC, NAP (Network Access Protection), or VLAN issues.
  • Check to make sure that cable is good, and its properly connected to the NIC and Switch.
  • Replace the cable or switch the cable to known good port on the switch.
  • Make sure other devices are able to communicate on the that switch.


Problems connecting on a Wireless Network

Below is a list of things that you can do to try to troubleshoot wireless networking problems:

  • Try utilizing a wired network connection and see if your problem goes away. 
    • If it does, then there is a problem with the wireless hardware or configuration on the local computer or wireless access point.
    • If it doesn't, it could mean that there is a problem with the computer network hardware, driver or configuration, there can still be a problem with the wireless access point or switch that you're using, or there could just be a problem with your ISP.
  • Some things to try:
    • Reset your network adapter
      • in the Control Panel open the Network and Sharing Center, and click the link for Change Adapter Settings.  Right-click the network adapter and select Diagnose.


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