Windows Hyper-V server has a default limit of 256 dynamic MAC addresses

Applies to: Windows Server 2008 DatacenterWindows Server 2008 EnterpriseWindows Server 2008 R2 Datacenter


Consider the following scenario:

You have a Windows Server 2012 computer which is configured with the Hyper-V role. The Hyper-V server is configured to provide Dynamic MAC addresses to the guest machines.  

When you start a guest virtual machine, you may encounter the following error message:

The application encountered an error while attempting to change the state of '<Virtual machine name>'

Synthetic Ethernet Port (Instance ID CCE417C5-BDD9-4216-85CA-248620EE75C6): Failed to power on with Error 'Attempt to access invalid address'.

On a Windows Server 2008 or Windows Server 2008 R2 Hyper-V host, an Event ID 12565 from source "Microsoft-Windows-Hyper-V-Worker" is logged, as described in the TechNet documentation.


This problem occurs because the default number of dynamic MAC addresses for virtual machines (256) has been exceeded. 

Hyper-V generates the MAC address as described below (mapping MAC address to aa-bb-cc-dd-ee-ff):
  • The first three octets (aa-bb-cc) are Microsoft's IEEE organizationally Unique Identifier, 00:15:5D (which is common on all Hyper-V hosts.
  • The next two octets (dd-ee) are derived from the last two octets of the server's IP address.
  • The last octet (ff) is automatically generated from the range 0x0-0xFF.

Because the last octet is an 8-bit value, there is a default limit of 256 possible MAC addresses.


Apply one of the following solutions:

a) Turn off the virtual machine, allocate a static MAC Address that does not belong to Hyper-V's dynamic MAC address range, then restart the virtual machine. 

b) Increase the range of MAC addresses by using one of the following methods, by modifying the fifth and/or the sixth octet of the default dynamic MAC address range:

If the host is Windows Server 2008, you can modify the dynamic MAC address range via the following registry values:

HKLM\Software\Microsoft\Windows NT\CurrentVersion\Virtualization\MinimumMacAddress
HKLM\Software\Microsoft\Windows NT\CurrentVersion\Virtualization\MaximumMacAddress

If the host is Windows server 2008 R2, you can use the Virtual Network Manager GUI, as documented in the following MSDN blog post:

Hyper-V R2: MAC Address Management

Note: Knowledge of the hexadecimal numbering system is helpful to modify the octets. Each octet's range is 00 - FF.

More Information

Please refer to the following blog posts for additional reading on this subject.

Hyper-V R2: MAC Address Management

Managing MAC Address Pools in Hyper-V R2