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IP address conflict

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NOTE: This document strongly reflects the writer's experiences with the most common business computing environments, Windows on PCs communicating over Ethernet.

An IP Address Conflict occurs when two devices on the same network claim to have the same IP Address - that is, they conflict with each other. Since only one of the devices is supposed to be on the network at a time, the second one to arrive will generally stop its IP functionality. In many cases with modern Operating Systems, the Operating System will notify the user of the device that there is an IP Address Conflict (displaying the symptom error message) and then either stop functioning on the network or function very badly on the network, and the user will then be stumped as to how to resolve the conflict. The following steps should help you to resolve the conflict.

  1. Document the initial symptom. Write down exactly (or print the screen containing) the entire "IP Address Conflict" error message. Many users experiencing the initial symptom will ignore it, hoping that it will go away (exactly the wrong approach for this symptom).
  2. Try to release and renew your IP Address (if using Windows, "ipconfig /release" followed by "ipconfig /renew"). If you get a different one, chances are the different one won't be in conflict.
  3. If you are still experiencing the symptom, you will be looking for the following five pieces of information:
    1. The IP Address that is in conflict.
    2. The identity of the device reporting the conflict, in layman's terms. This is generally your PC.
    3. The MAC Address of the device reporting the conflict.
    4. The MAC Address of the other device.
    5. The identity of the other device, in layman's terms. This is generally somebody else's PC or printer.
  4. Once you have 3.5 above, you should be able to do one of the following: get 3.5 to release and renew its DHCP IP Address (if Windows, "ipconfig /release" followed by "ipconfig /renew"); change 3.5's Static IP Address to one outside the DHCP Range (make sure no one else is using the new address first); or except 3.5's Static IP Address from the DHCP Range (requires a change to the configuration of the DHCP Server).
  5. If you have 3.4 above but not 3.5 above, you should inventory all of the Network Interface Controllers (NICs) in all of the devices on the network containing the brand of NIC identified by IEEE OUI and Company_id Assignments, or "ping -a " 3.1 above from a working Windows PC on the same network to try to determine the machine name of 3.5 above. Many Windows-compatible devices will report a machine name even if they are not running Windows.
  6. If you have 3.3 above but not 3.4 above, use "arp -a " 3.1 above from a working Windows PC or a local equivalent on the same network to display the ARP Cache for 3.1 above until you get a response different from 3.3 above.
  7. If you have 3.2 above but not 3.3 above, use "ipconfig /all" or a local equivalent to display 3.3 above. 3.3 above may also be displayed in the properties of your NIC in your "Windows Control Panel" named "Network Connections", printed on the back of your device near the network cable jack, and/or printed on the device's network card.
  8. If you do not have 3.2 above, interrogate the user who originally reported the symptom until they either reveal 3.2 or stop reporting the symptom.
  9. If you do not have 3.1 above, you may be able to get it from your control panel or from "ipconfig" or a local equivalent, but it's best to restart at Step 1 above.
  10. Now that the emergency is over:
    1. Enjoy a beverage of your choice.
    2. Don't drink and drive.
    3. Think about inventorying your local network so that you can later associate IP Addresses, MAC or Physical Addresses, Machine Names, and device identification in layman's terms, and not be quite so frustrated the next time this problem crops up. Tools that can help in this process from the Windows command line are "arp -a" and "ping" IP Address, as well as asking the DHCP Server what devices it's serving. Better yet, get the intern to do it. :)

Alternative Method

For an IP address conflict in Windows XP on a wireless router, if you have an idea of which computers are having a conflict follow these steps:

On each computer:

  1. Go to the 'Start' menu, select 'Run...', type in "cmd" and hit 'Enter' or press 'OK'
  2. In the command prompt window, type in "ipconfig" and hit 'Enter'
  3. Write down the IP address and Default gateway for each computer.

For the two computers with the conflicted (ie: shared) IP address, choose one computer and do the following:

  1. Go to 'Control Panel' and select 'Network Connections'
  2. Right-click 'Wireless Network Connection' and select 'Properties'
  3. Under the 'General' tab, in the box under the heading 'This connection uses the following items:', scroll down to 'Internet Protocol (TCP/IP)' and highlight it. Press the 'Properties' button
  4. Select the 'Use the following IP address:' option and input a new valid IP address that will not conflict with any other computers on the network, and input the same information as the other computers for the remaining two fields (Subnet Mask is automatic, and Default Gateway is the information you should have gathered before)
  5. Press 'OK' and the problem should be solved

If you still find you have troubles, go back and select 'Obtain an IP address automatically' instead of 'Use the following IP address'. In my experience, manually changing my IP address, then logging onto the network, and then allowing automatic setting of IP addresses have worked.

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