When setting up Hyper-V in Windows Server 2008 you need to setup network adapters for your virtual machines via the Hyper-V Network Manager (or during Hyper-V setup). The creation of these virtual network adapters will turn the adapters on the physical host into virtual switches and create new adapters on the physical host. The new adapters can actually be disabled if you don’t want the host to use them.
Example 1:
I setup a server with Windows Server 2008 running Hyper-V with three virtual servers running on it. The physical box had 4 NICs and I wanted one dedicated to the host system, and one dedicated to each of the virtual servers. On building the host I had Network Connection #1, Network Connection #2, Network Connection #3, Network Connection #4. After setting up the three virtual network adapters for Hyper-v: Network Connection #1 has no change, but Network Connection #2, #3, and #4 now didn’t have IP v4 or IP v6 selected and were acting as virtual switches. Additionally, there were new network adapters listed on the host. I disabled these new adapters because the host didn’t need to use them. The VM’s each had a NIC in their OS that got an IP address, but that didn’t mean that the host needed an IP on that adapter, because the VM is using the virtual switch and technically the additional adapters (the ones I disabled) were also using the virtual switch.
Example 2:
You could actually use this feature in a really cool way. Setup a server with Hyper-V and disable all network adapters on the host, then allow the VM’s to have access to the virtual switch so the VM’s can get out to the network. This way you could run multiple OS’s on a host server that never touched network. The host is protected and the VM’s can have a snapshot taken and reverted to a known working state when needed.