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My computer and that of a collegue have a changed their adress when booting up this morning. The only thing I can think of right now is the windows automatic update which has kicked in tonight to deliver and install KB2286198, which doesn't really talk about this.

Is anybody else seeing this behaviour or should I be worried something else is going on behind the scenes?

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Are you sure it's the MAC address that's changed and not the IP address? – Marko Carter Aug 4 '10 at 8:34
Yes, I am 100% sure it's the MAC address because my job involves writing software that checks certain things based on MAC address (lame authentication, don't ask...) and that's how I noticed. – BartV Aug 4 '10 at 9:27
Or someone is spoofing your connection and forwarding it after sniffing data, so the authentication system you're looking at is seeing where data is being repeated through...MAC addresses don't normally just "change" unless you're telling the OS to report a different one than the hardware has. Anything's possible though. – Bart Silverstrim Aug 4 '10 at 11:10

you don't have to worry ! If you MAC address has really changed the only direct issues I can immagine are wireless authentication based on MAC Adresses. If so, go to te admin interface and update mac addresses you allow to join the network.

May be you can also have dhcp / fixed ip issues. But if you don't use any of them, it's ok.

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A MAC address can be changed, see here but you have to go out of your way to do it. Check that a value hasn't been set in the registry.

The simplest explanation is that you're connecting to the network via another network card? Does the computer have more then one wired NIC? has a cable been swapped around? Have you gone from connecting to the network from wired to wifi, or vice versa?

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