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Duplicate MAC address

By Sachchi ·
Recentrly there was a LAN outage on one of the floor of our ofice. After troubleshooting the problem it was noticed that that there are three PCs talking with the same MAC address.

We bought two PCs and installed in a saperate test lab. We reinstalled fresh OS (WIN2K) and changed the Hard disk (to insure that there is no software based MAC address changes). But still MAC address of both PCs was same ! The Mercury (845) mother board of both the PCs has same MAC address ! The onboard NIC is of SiS900 Fast Ethernet.
How this can happen ? How the MAC address is assigned to the mother board ? is it soft burnt (EEPROM kind of) or is it hard coded into the ASIC chips ? At PC level, How should this problem be resolved & prevented?

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by BFilmFan In reply to Duplicate MAC address

The MAC address is usually hard-burned into the network cards ROM, but in some of the newer cards this can be set by the OS. You would have to reload the OS and manually change the MAC address.

This was quite common back in the old days (yah I know when the network geeks ran around in loin cloths and rode dinosaurs to work) when Token-Ring at 4 Mbps was King!

Details on changing the MAC address can be found here on an XP system can be found here:


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by Sachchi In reply to

As per the link you provided I checked the method-I&II. As per method-I there was no value change. As per method-II there was no "Network Address" in ?HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318}\006.

So there is no change of MAC at software/OS level. These PCs are having same hard coded MAC address!

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by PC_Kahuna In reply to Duplicate MAC address

So you have tried the link from the other answer to your post and it was no help. I would suggest entering the BIOS at start up and disabling the on board NIC and save the settings and power off the PC. Install a NIC in one of the free expansion slots and reboot. 10/100 NIC card are cheap.

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by Sachchi In reply to

Hi Mr Kahuna,

Sorry for being late to reply.. as I have changed the job.

We had done exactly same what you had adviced to resolve the problem on temp basis. However my question was "how this can happen ?".

Lately I had meeting with the vendor. After showing the tests and proofs of duplicate MAC address, they agreed that mistakenly the same MAC address was burnt into multiple motherboards by motherboard manuifacturers. They took back the motherboards and replaced by new one.

Sachchidanand Muchandi.

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by Sachchi In reply to Duplicate MAC address

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