Virtual Machines can use 2 different types of virtual Network.
You can create several virtual networks and also assign them to different virtual network ports.
Bridge is directly attached to one or several network ports. Through the bridge the virtual machines will get the direct access to the network.
This mode creates a virtual network that will capsule virtual machines into their own virtual network. They will communicate with each other and also have access to the outside world
NAT network can also provide a DHCP services for the VMs
On VM creation or edit you can then assign the networks to individual NICs.