Editing the MAC Address for Guests

Thursday, October 20, 2011

For those of you who use Microsoft Virtual PC in labs and/or training rooms, you may have reached this article because you probably have a duplicate MAC issue you are trying to resolve. Microsoft Virtual PC uses a .VMC file for storing some basic settings of the guest system. One of these configuration settings is the MAC address for the Ethernet adapter(s) configured for the guest system.

The .VMC file is formatted using XML notation. To find the MAC address for the virtual guest's Ethernet adapter, search for the tag called "<ethernet_card_address>". Within that tag, you will find the 12 digit hexadecimal number used for the MAC address. Keep in mind that the hexadecimal system using the values of 0-F for each place holder. If you copy the .VMC file to another computer for easy distribution, make sure that you change the MAC address on the target system(s) to ensure that you do not have any duplicate MACs on the local subnet. When duplicate MACs are present, delivery of packets will be inconsistent depending on which system holding the duplicate MAC responds first. You can watch this short video which follows through the process of updating the .VMC file.

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