virtualbox[.]org/
Note that this is my experience -- mileage may vary...
Installing 7.1 over the preexisting version, my Windows guest VMs did not have any network connectivity. I have the V/Box console set to advanced. Clicking the 3 line menu for Tools, I clicked the Network option to open the very basic network mgr. GUI. *Only* the "Host only" tab had an entry -- NAT Networks was blank. Clicking the Create button on the NAT tab gave me "NatNetwork" by default. In the settings for each VM I then had to go to the Network tab & set "Attached to" from NAT to the new "Nat Network".
In the VMs themselves, I have to install the new 7.1 Additions twice -- installing once does not enable the mouse integration, so you have hit the right Control key to release the mouse.