Hello AV, welcome to the Seven Forums.
This is most certainly because of the Power Settings of your host's NIC.
Go to Device Manager
(host), right click your NIC and choose Properties
. Click the Power Management
tab, uncheck the box Allow the computer to turn off this device to save power
. Click OK
Now your NIC does not sleep and virtual machines don't lose connection to it.
If you don't want to change NIC's power settings, you can always hibernate and restart the virtual machine after computer is back from sleep. This way the virtual machine connects back to to host's NIC.