When you install VirtualBox on a Windows host, the setup program installs a special networking driver on your system. This driver, the VirtualBox Host Interface NDIS driver, can be used to create additional host interfaces. These must be created explicitly before they can be attached to a virtual machine.

You can use the network configuration in the VirtualBox GUI to create and delete host interfaces on your Windows system.

Alternatively, use the VBoxManage tool:

VBoxManage createhostif "VM1 external"

Each new host interface thus created appears as an additional network card in your standard Windows "Network Connections" properties. After you have created your new host interface this way, you can select "Host Interface" as the networking mode in a virtual machine's Settings window and select the new interface in the "Interface name" drop-down list. With the above example, this drop-down list would contain "VM1 external".

If your host is running Windows XP or newer, you can also use the built-in bridging feature to connect your host interfaces to your physical network card. After creating the desired host interfaces, select your physical network adapter in the Network Connections folder and the desired host interface adapters and select "Bridge connections" from the popup menu. Note that you have to transfer your network configuration from your physical network adapter to the network bridge as mentioned above, because your physical network adapter will only function as a transport medium in your bridge setup. When more than one connection is active on a bridge, Windows will automatically put your physical Ethernet adapter into promiscuous mode so that it will receive network data for all bridged connections.