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IP Conflict in a Windows 2003 AD domain

We are running a Windows 2003 AD network.  We have about 50 users. I have an IP range of - available for DHCP

Occasionally I get users who report getting
"windows has detected an IP address conflict... Another computer on this network has the same IP address as this computer. Contact your network administrator for help resolving this issue.  More details are available in the Windows System event log.  "

Anyways, its not a huge issue, since it only happens once a week or so, and even then users just need to reboot and the are fine.  

Question is, how should I treat the problem.  Should I just assume that this will happen time to time? Or is this an issue that can be resolved.

4 Solutions
Normal (occasional) situation: This is an extract from an article (don't recall the url...

"IP address conflicts usually occur when someone turns off a computer with an expired IP address lease. Often, the DHCP server has already issued the IP address to another computer. The first computer requests to use its original IP address, and if the DHCP server doesn't respond quickly, the computer uses the last IP address the DHCP server issued it. Thus, two computers are using the same IP address, which creates a conflict."
ajwukTechnical Consultant / Project ManagerCommented:
How many mobile users do you have and are you using a standard class C network? I.E.
Often, if you've used a network range which is commonly used as the default for home user network devices ( and 1.0) this is where a lot of the overlap can come in.

Also, is there any pattern to the IP addresses in conflict? Sure no one has accidentally manually assigned an IP address which is in the DHCP scope?
It might be in your best interest to start adjusting your DHCP Lease times within your DHCP scope...

You always have your users out there that always keep their computers on... And of course the people that immediately turn their machines off after they're finished.

Certainly the people that keep their computers on will hold onto the IP Address DHCP gave them for whatever time you have the lease set to.

Hopefully depending on how smart your network infrastructure is, that once people log off or turn their computer off, the IP Address is released from DHCP, depending on Address Resolution Protocol, or additional config within DHCP. But on the flipside, some people like to maintain the MAC Addresses of computers within DHCP/DNS, so no matter how long a computer would stay off, DHCP would still assign the same address to that particular computer...

Or to go further, reserved DHCP IP Addresses based on MAC Address.... Which I personally like when it comes to smaller networks... Then you need not to worry about your IP Address range flipping all over the place as computers log on and off.. I know you have 50 users.... But that initial time to config would definitely resolve your issue, and have more control of your address assignments.

Of course this is all generalized information, but I agree with ajwuk to if there's a pattern to this....


I just wanted to add a couple of things what has already been said.

You don't mention the OS of the clients, but Windows Vista will throw up an error about an IP address conflict on the network, but then will automatically go through the DORA process to resolve the issue, negating the need for a restart. Windows 7 should have been changed to allivate the pop-up all together, or at least delay it until there is a real problem, but I haven't confirmed that.

The option of releasing the address on shutdown was mentioned, but a couple of things to know about that. First, the option is configured on the DHCP server at the scope or server options level under advanced options, but you have to use the Vendor class "Microsoft Windows 2000 Options" for this option to be passed to the clients. It’s also listed under “Microsoft Options” but doesn’t work here.

On the clients, APIPA has to be disabled for the option to release at shutdown to work properly. This is done by adding a DWORD value IPAutoconfigurationEnabled with decimal value 0 under the registry key HKLM\SYSTEM\CurrentControlSet\Tcpip\Parameters.

This option doesn't resolve the issue of laptops being put to sleep and then brought back in a week later, but if that is the issue, then lowering the lease times, as mentioned, is the best option.

There is also the option of increasing the IP conflict detection attempts on the DHCP server itself, but this can introduce startup latency on the clients. Perhaps setting it to 1 or 2 might help though and is worth testing.
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