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I was reading an article on WEP cracking, which said that filtering MAC addresses was not enough, since there are tools to change what the computer reports as its MAC address.

This made me wonder what would happen if someone uses the same MAC address as another computer on the network?

I've looked on Google, but most websites just say something like "strange things could happen". What kind of things? Packet loss, disconnected users?

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The tag for wifi didn't really apply because this question applies to computer networks in general. I think you'll get better results with the networking tag. –  Travis Aug 3 '09 at 17:29
Thanks, I didn't think that the question could be so generic. –  Manu Aug 4 '09 at 11:23
Its only generic in the sense that something like a MAC address can cause so much havoc. Some routers fare better at handling these things, but the MAC address ideally supposed to be a fundamental piece of the TCP/IP stack that it is somewhat assumed that it doesn't change or collide. –  Dillie-O Aug 4 '09 at 15:08

2 Answers 2

up vote 16 down vote accepted

It really does depend on how the routers and systems on the network are configured.

At our office, our machines will not connect to the local domain due to the collision in MAC addresses. You'll get a notification message (in Windows) saying there is already a system with the Id on the network.

Sometimes you get into "races" where each computer attempts to register itself with the router, and any traffic coming to the machine can get lost since packet A will go to your machine, the other machine will register, so packet B will go there. Things can start bouncing back and forth.

You can start seeing unreachable host errors due to the collisions as well.

The results really do vary depending on when the duplicate machine is coming online and how the current infrastructure is setup to handle such items.

Your network admin will have more detailed answers on this.

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+1 - it really depends on the devices that are dealing with the MACs. –  EvilChookie Aug 3 '09 at 17:04

Basically the expected behavior is "undefined".

Basically the two network cards would both present all packets sent to that address to their respective machines which will cause some confusion when both are active. I suspect that neither machine would be able to effectively communicate with the network while the other were active.

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