Hyper-V Dynamic MacAddresses

Doug Gabbard

In the Hyper-V Networking you speak of dynamic Mac Addresses and that hyper-v has a default range. If there are multiple hyper-v hosts do they use different dynamic ranges? If so, how is that managed. I would think they would have to be different since Mac Addresses are supposed to be unique.

Mike Rodrick

Great question Doug.

This is definitely something you have to plan for when implementing multiple Hyper-V hosts.

Microsoft has reserved the OUI (the first 6 hex digits of the MAC address) 00-15-5D. Both the minimum and maximum address in the pool will begin with this. The last two hex digits of the minimum will be 00 and the last two for the maximum will be FF. The remaining 4 digits, (00-15-5D-XX-XX-[00-ff]) are determined by the third and fourth octet of the host's IPv4 address, converted to hex.

So if the Hyper-V host has an IPv4 address of 172.16.5.201, the MAC Address Range will be:
Minimum: 00-15-5D-05-C9-00
Maximum: 00-15-5D-05-C9-FF

If a second Hyper-v host had an IPv4 address of 172.16.7.210, the MAC Address Range will be:
Minimum: 00-15-5D-07-D2-00
Maximum: 00-15-5D-05-D2-FF

This helps to reduce the chance of conflicting MAC addresses. These values are stored in the registry under HKLM\SOFTWARE\Microsoft\Windows NT\CurrentCurrentVersion\Virtualization\MaximumMacAddress and MinimumMacAddress. However, these keys are not updated if the IPv4 address on the Hyper-v host is changed. This can lead to overlapping pools and conflicts, as can reusing IP addresses on Hyper-V hosts.

You can remove the MaximumMacAddress and MinimumMacAddress registry keys after updating the Hyper-V host's IPv4 address, restart the Hyper-v service, and the keys will be recreated using the new address as a base. You can also manage the pool of MAC addresses manually.

To manage the MAC address range for a Hyper-V host, open the Virtual Network Manager. In the left pane, you will see Global Network Settings and MAC Address Range. By selecting MAC Address Range, you will see the Minimum and Maximum values for the MAC Address Range for that Hyper-V host. You will need to change these values appropriately on each Hyper-V host, so the ranges do not overlap.

If virtual machines have already been created, and you modify the MAC Address Range, you will need to remove and re -add the virtual network adapter in the VM to have a new MAC address assigned. Newly created VMs will be assigned a MAC address from the new pool.

Mike