If you want to guarantee that the same MAC address is assigned to a given virtual machine every time, even if the virtual machine is moved, or if you want to guarantee a unique MAC address for each virtual machine within a networked environment, you can assign the address manually instead of letting GSX Server assign it automatically.

To assign the same, unique MAC address to any virtual machine manually, use a text editor to remove three lines from the configuration file and add one line. The configuration file has a.vmx extension at the end of the filename. On a Linux host, a virtual machine created with an earlier VMware product may have a configuration file with a .cfg extension.

Remove the three lines that begin with the following:

ethernet[n].generatedAddress
ethernet[n].addressType
ethernet[n].generatedAddressOffset

In these options, [n] is the number of the virtual Ethernet adapter — for example ethernet0.

Add the following line to the configuration file :

ethernet0.address = 00:50:56:XX:YY:ZZ

In this line, XX must be a valid hexadecimal number between 00h and 3Fh, and YY and ZZ must be valid hexadecimal numbers between 00h and FFh. Because GSX Server virtual machines do not support arbitrary MAC addresses, you must use the above format.

So long as you choose a value for XX:YY:ZZ that is unique among your hard-coded addresses (where XX is a valid hexadecimal number between 00h and 3Fh, and YY and ZZ are valid hexadecimal numbers between 00h and FFh), conflicts between the automatically assigned MAC addresses and the manually assigned ones should never occur.

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QUELLE: http://www.vmware.com/support/gsx3/doc/network_macaddr_gsx.html