If we want to create virtual machines that have to communicate between them or have access to internet, we must create a virtual switch.
For this particular case, I want to create a internal virtual switch to allow the communication only between a group of VMs that will be connected to this switch. We can also create a external virtual switch if we want to connect directly to a network card.
At Hyper-V Manager window, click/select your computer on the left pane and then click on Virtual Switch Manager… on the right pane.
Select Internal and click Create Virtual Switch
Type a name for the new virtual switch, click OK and a new virtual switch will be created.