How dynamic media access control addresses are assigned to virtual network adaptors in Virtual Server 2005

This article describes how dynamic media access control (MAC) addresses are assigned to virtual network adaptors in Microsoft Virtual Server 2005.

The MAC addresses that are assigned to virtual machines in Virtual Server 2005 are allocated according to the following algorithm:

1. The first three bytes of the MAC address are the Microsoft Virtual Machine Organizational Unique Identifier (OUI). The first three bytes appear as 00:03:FF.
2. The fourth byte is the fourth byte of the MAC address of the host adaptor in an XOR (exclusive OR) relationship with a monotonically increasing counter that is maintained by the Virtual Machine Network Services driver.
3. The fifth and sixth bytes are the fifth and sixth bytes of the MAC address of the host adaptor. If there are multiple host adaptors, the first 256 addresses are allocated from the first host adaptor, and the next 256 addresses are allocated from the second host adaptor. When all host adaptors have been used, the algorithm re-uses the first network adaptor.

Note The Microsoft Loopback and Bridge adaptors are not considered for the purposes of the MAC address allocation mechanism. Typically, these adaptors have the same MAC address on every computer. If there are no eligible host adaptors, the last three bytes of the MAC address are chosen at random by using a crypto random number generator.

Important This information applies to the current version of Virtual Server 2005. Microsoft reserves the right to change the algorithm in future software updates and in other product releases.

There is no guarantee in Virtual Server that MAC addresses that are dynamically allocated to Virtual Machines across multiple Virtual Server hosts will be unique. This is because there is no "central" MAC address pool from which multiple hosts can pull.