When MAC Addresses collide....

Soul Rider · 25 Mar 2006 at 15:34

OK, I've done my A+, my partner in crime who runs our LAN is N+ certified and most of our regulars are IT Professionals in one capacity or another, and we can all confirm this, and will be adding video evidence at the next LAN..

The chances of this happening must be pretty slim, but anyway here goes...

2 of our regulars set-up their machines, and they were having wierd problems with their LAN connections. We were investigating both machines when we discovered the impossible, they had identical MAC Addresses.

Now this is supposed to be impossible, infact it's part of the A+ training that states the first 24bits of the MAC address are for Manufacturer identification, and the second 24bits are for Card identification, to ensure there is no duplication.

So what is the chance of this happening? And is it worth getting all the serial numbers from the boards and everything and mailing the manufacturer to let them know that:

A: this has occurred
B: that there may be a whole batch of motherboards out there with the same onboard NIC supplying the same MAC address to people.

Anyone else ever experienced this?

tolien · 25 Mar 2006 at 15:57

Most common cause for it I've ever seen was BIOS flashes breaking the MAC addresses - check that what they're using is what they're meant to be (the ones in question had labels on the boards with the LAN/Firewire MAC addresses).

MAllen · 25 Mar 2006 at 17:48

I used to work for a network hardware company who produced Network Printservers. And yes you are right that the MACs should be unique as they leave the factory. Ethernet relies on these being unique to know where to direct it's packets.

Test boards would often pop-up on our own internal network with test addresses that often clashed - causing chaos to us trying to develop the software.

The stuffed BIOS idea sounds right. Harrass the manufacturer as they should be able to give you a utility to change your MAC address. There is probally a tool for updating the MAC in the BIOS.

Normally the box and/or the motherboard itself will also have a sticker on it with the MAC address that was shipped from the factory.

A quick fix would just be to use a PCI NIC.

Curiosityx · 25 Mar 2006 at 18:06

System Specs? Specifications on the network adapters? Using a switch, hub or connected directly? On board design or Pci based? Speed?

Yes there is a extreamly small chance of this happening but it can, even though vendors use a list of used and un-used current Mac addresses that have been hard coded onto the card to avoid conflictions.

There are plenty of tools that are publicaly avaliable to change the Mac on a particular card.

Soul Rider · 26 Mar 2006 at 09:46

These were both brand new motherboards that had been bought and installed on different systems, by different people a few months apart.

The LAN is running on an Ethernet Modem/Router plugged into a 32 port unmanaged switch. The set-up was fine, the problem occurred as it was the first time one of the rigs had been brought along, as the guy had just built it.

There has been no bios flashes on either of them. I can't remember what boards they are, just that they are nForce4 boards.

At our next LAN we are going to dig into it further and get some video evidence to send off to the manufacturer.

The fix used on the day was just to install a NIC, but I was more concerned by the fact it isn't supposed to happen..

V-Spec · 26 Mar 2006 at 10:59

NIC factorys have had problems with faulty mac assigning hardware in the past, there have been instances where a bunches of cards received the same mac address, and sold on before the problem was known, obviously this is only a problem on a LAN, we did have the problem in a previous job, we bought 2 brand new hp desktops, the desktop guys were busy ghosting them and complained that 2 were having problems, turned out they both had identicle mac addresses. Spoke to HP and they were aware that a fault had occured at some time in the production line...

MAllen · 26 Mar 2006 at 13:12

In a world of machines, we still have Human Error.

Someone hit the wrong switch at the factory.

subroutine · 26 Mar 2006 at 14:53

tolien said:
Most common cause for it I've ever seen was BIOS flashes breaking the MAC addresses - check that what they're using is what they're meant to be (the ones in question had labels on the boards with the LAN/Firewire MAC addresses).

This happened with my MSI Neo2. The mac address was located on the printer socket.

.

Soul Rider · 26 Mar 2006 at 15:21

So it's obviously not as rare an occurrence as I was led to believe....

Well at least that clears that up. Will be interesting to check if everything else is fine when we meet up for the next one.

Skiddley · 27 Mar 2006 at 09:05

I thought the last 3 octets where user definable and/or you can override the BIA with an LAA? Amazed if they are not unique out of the box!

Vertigo · 28 Mar 2006 at 18:50

Only time I've ever had a MAC conflict was when I first had cable installed at home, back when MAC addresses needed registering.

The guy installing it said that my address was already registered in the database which I told him was impossible as the NIC was brand new and had been in the shrinkwrap a day earlier.