At some point in the course of being a network admin, you have had to track down a virtual machine on the network. This task can be complicated, as clients may (legitimately) be running a virtualization platform on their client systems. Knowing what platform a MAC address relates to may help in identifying the offending system(s). Here is a rundown of common MAC address organizationally unique identifiers (OUI) and their associated platforms to help with the identification of virtual machines on the network:

Company and Products MAC unique identifier (s)
VMware ESX 3, Server, Workstation, Player 00-50-56, 00-0C-29, 00-05-69
Microsoft Hyper-V, Virtual Server, Virtual PC 00-03-FF
Parallells Desktop, Workstation, Server, Virtuozzo 00-1C-42
Virtual Iron 4 00-0F-4B
Red Hat Xen 00-16-3E
Oracle VM 00-16-3E
XenSource 00-16-3E
Novell Xen 00-16-3E
Sun xVM VirtualBox 08-00-27

Note that a lot of the products use the same format, namely the ones based on the Xen hypervisor and its roots in the open source community. Having this information available can give you a hint to the possible destination. Further, to manage your connections for development teams, you may seek DHCP reservations for MAC addresses of virtual machines that are used by clients to isolate networks.

Network management software may allow the client to specify a MAC address within the operating system. For example, within physical adapters, certain versions of the Intel teaming software would permit the user to specify a virtual MAC address within the software. These software titles may not function the same way with a virtual network adapter but may permit a mix of one physical and one virtual adapter in the case of a physical-to-virtual conversion. Further, some of the products allow various levels of MAC address modification, but the above list constitutes the default OUIs for the platforms.