If you like your Hyper-V story to run smoothly your server needs two network cards – my HP DL380 has two gigabit cards – one is used for Hyper-V server to have internet connectivity (updates, remote management …) second one is given to Virtual network manager to be used by virtual machines.
If you did not select any network adapters to be used by Hyper-V during Hyper-V installation you can see your physical two adapters in Network connections.
After you run Virtual network manager and you create new network for example “External network” that is connected to your second physical card your physical card lose connectivity and all protocols and services are unchecked except Microsoft Virtual Network Switch Protocol (and HP Network configuration utility – but this is specific to my case becouse I use HP server). When you finish the wizard you can see that in your Network connections there is another adapter called “External network” – it is now used for your Virtual machines.
HINT! Do not do that remotely if you are not completely shure which adapter is which… If you make a mistake you will need physical access to the machine to change network parameters. 
Example:
Hyper-V virtual network manager example [docx]
Hyper-V virtual network manager example [pdf]
