Recently, TechRepublic member Dan Buonocore, system support specialist at the City County Credit Union in Margate, FL, was having trouble with an IP address conflict message that kept appearing on several of his machines. To help solve the mystery, he posted a question in TechRepublic’s Technical Q&A and asked other members to help him find a solution.

Buonocore received advice from dozens of members, and the information aided him in narrowing down the problem. Each suggestion became part of a troubleshooting checklist that enabled him to eliminate certain options. In this article, we’ll explain Buonocore’s IP address mystery and how he is working to find a solution.

The interface has been disabled
Buonocore explained that he was managing a network with approximately 120 computers running Windows 95. Every six weeks or so for the past few months, a random computer would get an error message that said The system has detected a conflict for IP address _____ w/ the system having hardware address _____. The interface has been disabled.

“One day the computer would be fine, and then it would get that conflict. I tracked it down to another computer and released both the IP addresses, and for some reason, it kept on grabbing that one address. It wouldn’t let go of it,” Buonocore said.

After trying several things to no avail, Buonocore said he replaced the network card, which solved the problem temporarily. However, the conflict would eventually reappear on a different computer some weeks later.

Using the ARP command
Bill_H suggested Buonocore use the Address Resolution Protocol (ARP) command to clear the ARP cache. (According to Newton’s Telecom Dictionary, ARP is a low-level protocol within TCP/IP that maps IP addresses to corresponding Ethernet Address. ARP is used to obtain the physical address when only the logical address is known.)

The ARP cache contains the most recent list of IP addresses and the unique Media Access Control (MAC) address embedded in each network adapter. “I looked in the ARP cache, and that address wasn’t in there, but I cleared it anyway,” Buonocore said. While it didn’t solve the problem, this was now off his checklist and one less thing to worry about.

Who’s address is that?
RGershbock helped to shed a little more light on Buonocore’s problem. He suggested using a DOS command ( nbstat –A <ipaddress> ) to see the NetBIOS name of the computer and to check both machines to make sure they were using DHCP. He also suggested that Buonocore check to make sure that there were no overlapping address ranges if Buonocore was using two servers.

Again, Buonocore said he followed the instructions, and the time between incidents did seem to go down.
TechRepublic articles on TCP/IP and DHCP:“Configuring your workstations for DHCP”“Installing DHCP Service on your Windows NT server”“Download this primer for a solid foundation in TCP/IP”“Understanding new DHCP features in Windows 2000”From Microsoft:“Duplicate IP Address Detection”“Resolving Duplicate IP Address Conflicts on a DHCP Network”“Error Message: The System Has Detected a Conflict…”
The fix surfaces
Buonocore said that if he hadn’t gone through the steps provided by TechRepublic members, he wouldn’t have recognized the answer to his problem in the February 2001 issue of Windows 2000 Magazine’s Reader to Reader column. The reader suggested these steps:

  1. Go to the computer identified by the MAC address in the error message and clear the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet
    \Services\VxD\DHCP and DHCP Option subkeys, leaving only the PopuFlag and Version Values.
  2. Reboot the system and run WINIPCFG, select the NKI, and perform a Renew All. The system is now missing the default gateway information, so it should grab a valid IP address and DHCP information.
  3. Reboot the system with the error message and it will grab a valid IP address. The reader also suggested using the DHCP Manager to delete the IP address that was in retention.

Buonocore said he has not tried this yet, as he waiting for the error to make its next appearance—which, he said, should be any time now.
How did you fix it? We look forward to getting your input and hearing your experiences regarding this topic. Join the discussion below or send the editor an e-mail.