When a virtual machine is powered on, GSX Server automatically assigns each of its virtual network adapters an Ethernet MAC address. MAC stands for media access control. A MAC address is the unique address assigned to each Ethernet device.

The software guarantees that virtual machines are assigned unique MAC addresses within a given host system. In most cases, the virtual machine is assigned the same MAC address every time it is powered on, so long as the virtual machine is not moved (the path and filename for the virtual machine's configuration file must remain the same) and no changes are made to certain settings in that file.

In addition, GSX Server does its best, but cannot guarantee, to automatically assign unique MAC addresses for virtual machines running on multiple host systems.

To avoid changes in the MAC address automatically assigned to a virtual machine, you must not move the virtual machine's configuration file. Moving it to a different host computer or even moving it to a different location on the same host computer changes the MAC address.

You also need to be sure not to change certain settings in the virtual machine's configuration files. If you never edit the configuration file by hand and do not remove the virtual Ethernet adapter, these settings remain untouched. If you do edit the configuration file by hand, be sure not to remove or change the following options:

ethernet[n].generatedAddress
ethernet[n].addressType
ethernet[n].generatedAddressOffset
uuid.location
uuid.bios
ethernet[n].present

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

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.