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Resolving an IP address conflict

Posted on 2009-07-16
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Last Modified: 2012-05-07
Good morning,
We have a simple network of about seven (7) servers, all Windows Server based (all are 2003, except one which is 2008).  When we log onto our web server (Windows Server 2003) as an admin, we get a Windows System Error that says "There is an IP address conflict with another system on the network."

Things seem to working okay, but the message concerns us.  
How would we first go about identifying which addresses conflict?

Thanks for your help.
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Question by:cclausen1
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Bill Bach earned 1000 total points
ID: 24869549
On a TCP/IP network, the addresses have to be maintained by the system admins -- that's you.  Addresses can be doled out in two ways -- manually or automatically.  Manual methods involve assigning an IP address and then keeping track to make sure that you don't assign it twice.  The Automatic method most commonly used is DHCP, in which you configure a block of addresses to be assigned to users, and then allow the DHCP service to assign them.  A common mistake is in allocating a permanent address manually that exists in the DHCP range.

To resolve:

Get yourself a big sheet of paper, and start visiting each computer.  Run IPCONFIG on each one and look for sections like this:

Ethernet adapter Local Area Connection:
   DHCP Enabled. . . . . . . . . . . : Yes
   IPv4 Address. . . . . . . . . . . : 192.168.1.27(Preferred)
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   DHCP Server . . . . . . . . . . . : 192.168.1.11
(There will be more data, but these are the crucial items.)

Record the DHCP Enabled setting, the IP address, and the DHCP Server (just in case you have more than one) for each machine.

Chances are that one of the following is true:
1) You have more than one DHCP server.  Disable DHCP on the renegade device.
2) You have manually assigned an address in the DHCP range.  Modify your DHCP server settings to either reserve the address or shrink the address pool size to avoid the duplicate reservation.  
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Expert Comment

by:Qlemo
ID: 24870091
In Event Log (eventvwr) you should see which MAC address or even PC is claiming to have the same IP address.
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Assisted Solution

by:tlmacr
tlmacr earned 1000 total points
ID: 24997809
As others have stated, you may have an additional DHCP server that is leasing IPs and you just don't know about it, or the machine which raises the conflict has a static IP address and the DHCP server (if you have one) is giving out that lease to another machine when it should not be doing so.

If you don't have DHCP at all, and since its a small amount of machines, I would start by unplugging the rest of the machines and leave just the one that raises the conflict on the network.  See if the error still appears.  If the error appears, you might have duplicate assignments on the same machine, if it has multiple interfaces?

If the message doesn't appear, bring the other machines on one by one.  When you do so, check the machine which raises the error.  Anything that is going to cause a conflict I have found will appear almost immediately onscreen.  Once you have found the offending device then you have your answer and can then take appropriate action from there.

Good luck
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