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In an environment with virtual servers, is the MAC address used to communicate, the MAC address of the physical host, or the MAC address of the virtual adapters. If it's is the virtual MAC address how is this translation to the physical MAC address handled. I'm assuming that traffic will be transmitted as if it is emanating from the physical host. If that assumption is true, how is the translation back to the virtual MAC address typically handled?

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The MAC address of the virtual server is used throughout; typically the host uses transparent bridging to forward packets to the virtual adapters it is hosting. As is normal for a bridge, it forwards packets without changing the MAC address.

It's also possible to connect virtual servers via virtual routers, NAT, load balancers etc. In that case you may not see the virtual machines MAC on the physical network.

Open vSwitch is an example of a virtual switch implementation.

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    Just so I completely understand..To take an example, VMWare would provide a virtual switch which handles packets which are destined for the virtual MAC addresses.
    – Nattrass
    May 14, 2015 at 17:26
  • Yes. The VMs will be connected to the network via virtual switch, which will allow other devices to see a unique MAC address for each VM. Another scenario would be where you connect your machines via NAT, which would NOT expose the mac addresses of the VMs to the rest of the network.
    – IceMage
    May 14, 2015 at 17:55
  • @IceMage Good point, I'll update to make it clear that a transparent bridge isn't the only possibility.
    – richardb
    May 14, 2015 at 18:04
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To give you a very simplified answer: The physical NIC in the host is acting as a "bridge" or "passthrough" between the virtual switch and the physical network. There's no virtual MAC to physical MAC mapping whatsoever. For a virtual switch that isn't also being used for management of the hypervisor itself, there doesn't even need to be an ip address assigned to the physical NIC on the host that the virtual switch is bound to. From the perspective of the physical network, the virtual switch would appear as just another downstream switch in the sense that the MAC addresses of the virtual machines are all available through a single physical switch port (the port that the host physical NIC is connected to), just as physical machines connected to one switch are all available through a single port from any other downstream or upstream switch.

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