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I have a system with three NICs. The MAC addresses of the NICs are:

1) 2:208:201:168:25:72

2) 0:208:201:168:25:72

3) 0:208:201:168:25:71

I put NICs 1 and 3 into a bridge and Windows reports the MAC address of the bridge as 2:208:201:168:25:71.

My questions are:

1) What does a MAC address of a software bridge even mean?

2) How is Windows computing what the MAC address of the bridge? It looks like it is a combination of the MAC addresses of 1 and 3 somehow, but I would like to know exactly how it is being calculated if anyone knows.

The reason I care about this is that the generated MAC address of the bridge (2:208:201:168:25:71) actually conflicts with another NIC on a different machine on my network and is causing problems.

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Why are you telling is the hardware addresses in decimal? That is pretty unusual. – Zoredache Mar 30 '11 at 19:15
How have you managed to have the same MAC address on two interfaces on a single system? You mention a conflict, are you sure the system isn't conflicting with itself? You don't have all 3 ports plugged into the same switch/vlan do you? You haven't done something silly and disabled spanning tree on your switches? – Zoredache Mar 30 '11 at 19:18

According to this article, the Windows XP bridging feature identifies itself as the lowest-numbered MAC address of the NICs in the bridge.

To your credit though, it looks like it chose the lowest excluding the first segment.

Does it give you an option in the bridge properties to change the MAC address? The only requirement for a MAC address is that it is unique across ARP-able entries for the networking to work, so you could (if it allows you) change that MAC address to anything that you like that is not on your network currently.

The other option would be to possibly change the MAC address of the NIC on the other machine if that one is not part of a bridge.

Hope this is helpful.

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