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Trying to Understand MAC Address with a VM Machine

Posted on 2009-05-07
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Last Modified: 2013-11-09
How does a VM machine change the MAC addresses on a single NIC?  Does the NIC have multiple MAC addresses at any given time?  Is the VM swapping out MAC addresses based on the needed address.  Or, does it work some other way?
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Question by:jdana
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sryckman earned 336 total points
ID: 24330545
Remember that the VM does not have a physical NIC.   You may have mapped the virtual machine's virtual NIC to a physical NIC - but in the virtual machine it's just software.   The physical NIC on the machine just acts as a switch and forwards the traffic.   Thus the MAC address for a VM can be whatever the VM wants it to be.
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by:ashwin_kumar525
ashwin_kumar525 earned 332 total points
ID: 24330920
The MAC address will be dynamically generated for each VM you mount. You can also set it to static if you want. This is not only for VMs, we can also change the MAC for your physical machine using third party softwares.
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by:ranit8
ranit8 earned 332 total points
ID: 24364859
"The physical NIC on the machine just acts as a switch and forwards the traffic."

What sryckman said is right, and there are 2 possible ways, depending on the implementation:

Either the host works as an ARP PROXY so the external network machines send traffic directed to the VM guests's IP address to the host's MAC address, and the host sends it to the VM with the VM's MAC address as destination (much like NAT, but a layer below).

Or the host physical NIC is set at promiscuous mode, so whenever the address of the received frame matches the physical MAC or one of the virtual machines MAC, the frame is processed accordingly.
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Author Closing Comment

by:jdana
ID: 31579207
sryckman - Thanks for the clear summary.  
ranit8 - Thanks for the definition of ARP PROXY and promiscuous mode.  Both concepts are now in my notes.
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