How to Troubleshoot Duplicate Media Access Control Address Conflicts

If Windows detects a duplicate media access control address on the network, you will receive one of the following messages in the System event log: -or- Note: IP addresses and addresses will be different than the sample.

All network connectivity over TCP/IP to this computer will cease until the conflict is resolved and the system is restarted.

Media access control addresses are preassigned and permanently burned into the network interface card (NIC). These addresses under normal circumstances are always unique, however, rare errors made during the manufacturing process can cause duplicate media access control addresses to be used and cause this problem. Another source of duplicate media access control addresses can occur if you are assigning locally administered media access control addresses (LAA), in which case you are overriding the burned-in address in favor of the locally assigned media access control address. With some drivers, LAA media access control addresses are possible, usually Token Ring adapter drivers.

How to Troubleshoot

To troubleshoot this problem, you have to determine which other computer on the network is using the same media access control address. Several tools that ship with the Microsoft TCP/IP stack can be used to locate the duplicate addressed computer.

To isolate the duplicate media access control address, perform the following steps:

From a working TPC/IP based client:

1. Ping the TCP/IP address found in the event log entry from a command prompt by typing the following:
   PING 129.0.0.1
   
   You should get replies back similar to these from the duplicate addressed computer:

         Pinging 129.0.0.1 with 32 bytes of data:
   
         Reply from 129.0.0.1: bytes=32 time=10ms ttl=128
         Reply from 129.0.0.1: bytes=32 time=<10ms ttl=128
         Reply from 129.0.0.1: bytes=32 time=<10ms ttl=128
         Reply from 129.0.0.1: bytes=32 time=<10ms ttl=128
   					

2. To verify the computer's media access control address is the duplicate, type the following at a command prompt:
   ARP -a 129.0.0.1
   
   You should get a reply back as follows:

         Internet Address  Physical Address
         129.0.0.1 02:A0:8C:DE:00:FD  <-- matches the event log entry
   					

3. We can now use NBTSTAT to get the NetBIOS (friendly) name of the duplicate computer by typing the following at a command prompt:
   NBTSTAT -A 129.0.0.1
   
   You should get a reply back with the NetBIOS name of the computer. Use this NetBIOS name to determine who the owner of the computer is so you can locate it on your network.

      NAME             TYPE               STATUS
      -----------------------------------------------
      NTSERVER1           <00>                Unique
      DOMAN-NAME          <00>                GROUP
      NTSERVER1           <03>                Unique
   
      Media access control address = 02-A0-8C-DE-00-FD
   						

   If you get a message:
   HOST NOT FOUND.
   This would indicate that the duplicate computer is not a NetBIOS enabled computer, like a Novell server, Unix server, Router, or perhaps a Jet Direct Printer.
4. Once you find the duplicate addressed computer, you can either replace its network card, or if a locally administered media access control address (LAA), change it to be unique on the network.

Changing a LAA can be accomplished by performing the following steps:

1. In Control Panel, double-click the Network icon.
2. While displaying the properties of the installed network adapter, change the properties or configuration of the installed Network Adapters LAA to use a unique LAA.