Networking in Hyper-V is one of the main feature in a virtual infrastructure.
like physical dimension, for each virtual machine ( for each virtual NIC) assigned a MAC address (classic and logic networking).
The question is:
– How these MAC addresses are created ?
– How these MAC addresses are assigned to Virtual machines ?
– How can i avoid MAC addresses collision ?
– Static Mac addresses or Dynamic MAC addresses ?
John Howard -MSFT has written an excellent blog about how MAC addresses are generated and assigned in Hyper-V.
What can we learn from John:
1- MAC addresses are generated based on the Microsoft IEEE Organizationally Unique Identifier 00-15-5D and the last 16 bytes from the Host IP address: MAC address RANGE = 00-15-5D-(Hex(last 16 bytes of Host IP))+00 to 00-15-5D-(Hex(last 16 bytes of Host IP))+ff
Example:
If my host IP address is 192.168.20.1, the last 16 bytes are 20.1, in hex 14-01 and the MAC address range is [00-15-5D-14-01-00;00-15-5D-14-01-ff]
2- How MAC addresses are assigned: Each new created virtual machine NIC receives the next available MAC address.
Example: (NIC1, 00-15-5D-14-01-00), (NIC1, 00-15-5D-14-01-01), (NIC1, 00-15-5D-14-01-02) …
3– If MAC addresses range is saturated: Virtual machine with virtual NIC will be unable to boot
4- Dynamic or Fixed MAC range:
- In production environment, it’s recommended to fix your MAC addresses range. This will simplifie management, tracking, avoid collision.
- If your environment is large, and it’s possible to have two hosts with the same last 16 bytes, think to fix your MAC addresses range
- Try to manage your Virtual environment with SCVMM, it automatically avoid MAC collision, it has global view of all your hosts.
buildwindows 
Asking questions are in fact good thing if you are not understanding anything fully,
except this piece of writing gives pleasant understanding yet.