O modo em ponte é necessário para que as Máquinas Virtuais se comuniquem entre si e com o mundo externo
6.7. Host-only networking Host-only networking is another networking mode that was added with version 2.2 of VirtualBox. It can be thought of as a hybrid between the bridged and internal networking modes: as with bridged networking, the virtual machines can talk to each other and the host as if they were connected through a physical Ethernet switch. Similarly, as with internal networking however, a physical networking interface need not be present, and the virtual machines cannot talk to the world outside the host since they are not connected to a physical networking interface.
Instead, when host-only networking is used, VirtualBox creates a new software interface on the host which then appears next to your existing network interfaces. In other words, whereas with bridged networking an existing physical interface is used to attach virtual machines to, with host-only networking a new "loopback" interface is created on the host. And whereas with internal networking, the traffic between the virtual machines cannot be seen, the traffic on the "loopback" interface on the host can be intercepted.
Host-only networking is particularly useful for preconfigured virtual appliances, where multiple virtual machines are shipped together and designed to cooperate. For example, one virtual machine may contain a web server and a second one a database, and since they are intended to talk to each other, the appliance can instruct VirtualBox to set up a host-only network for the two. A second (bridged) network would then connect the web server to the outside world to serve data to, but the outside world cannot connect to the database.
To change a virtual machine's virtual network interface to "host only" mode:
either go to the "Network" page in the virtual machine's settings notebook in the graphical user interface and select "Host-only networking", or
on the command line, type VBoxManage modifyvm "VM name" --nic hostonly; see Section 8.8, “VBoxManage modifyvm” for details.
Before you can attach a VM to a host-only network you have to create at least one host-only interface, either from the GUI: "File" -> "Preferences" -> "Network" -> "Host-only network" -> "(+)Add host-only network", or via command line with
VBoxManage hostonlyif create see Section 8.33, “VBoxManage hostonlyif” for details.
For host-only networking, like with internal networking, you may find the DHCP server useful that is built into VirtualBox. This can be enabled to then manage the IP addresses in the host-only network since otherwise you would need to configure all IP addresses statically.
In the VirtualBox graphical user interface, you can configure all these items in the global settings via "File" -> "Preferences" -> "Network", which lists all host-only networks which are presently in use. Click on the network name and then on the "Edit" button to the right, and you can modify the adapter and DHCP settings.
Alternatively, you can use VBoxManage dhcpserver on the command line; please see Section 8.34, “VBoxManage dhcpserver” for details.
Note On Linux and Mac OS X hosts the number of host-only interfaces is limited to 128. There is no such limit for Solaris and Windows hosts.