virtualbox.org

When you set up an internal network in VirtualBox 4.0.4, it seems to work by default like a hub, so that all unicast traffic can be captured from a packet sniffer installed on any guest.

Is it possible to configure the virtual switch to work like a switch and set up private circuits rather than working like a hub?

Unless other VMs need to be able to access the two VMs as well, give the connections different names. E.g. VM1 and VM2 are connected to intnet1, VM3 and VM4 are connected to intnet2. This is of course only when VM1 or VM2 doesn't need access to, or be accessed by, VM3 or 4.