Chris: Adam, you raise some very good issues. Virtual MAC address assignments are based on Microsoft’s organizationally unique identifier (OUI) registration with the Institute for Electrical and Electronic Engineers (IEEE). An OUI is a unique MAC address prefix assigned that is assigned to all NIC vendors, both physical and virtual. You can check on the OUI of any vendor by pointing your Web browser to the IEEE Registration Authority – IEEE OUI and Company_id Assignments site.

If you check the IEEE site, you’ll see that Microsoft has the MAC address OUI prefix of 00-03-FF. So by default, Virtual Server VM MAC addresses will range from 00-03-FF-00-00-00 to 00-03-FF-FF-FF-FF. Here are the MAC OUIs of other popular virtualization platforms:

• Virtual Iron: 00-0F-4B
• VMware: 00-0C-29 and 00-50-56
• XenSource: 00-16-3E

By default, Virtual Server generates dynamic MAC addresses for each VM’s virtual network interfaces. Generating addresses dynamically when a VM starts will prevent duplicate MAC addresses from causing virtual network conflicts. Of course, if you’re using MAC address filtering on your network switches, then you'll need to assign static MAC addresses to each VM. To change a MAC address from dynamic to static, you just need to follow these steps in the Virtual Server Management Web page:

1. Under the Virtual Machines menu on the left side of the wide, mouse over “Configure” and then select the VM to configure.
2. Next, scroll down and click on the Network Adapters link.
3. For each virtual network adapter listed, click the Static radio button, enter a MAC address (if you want to change the existing address), and then click OK.

To help, read the original article, here, to find a VBScript to help create a list of all the assigned virtual NIC MAC addresses on your host server.