This allows VirtualBox to intercept data from the physical network and inject data into it, effectively creating a new network interface in software. When a guest is using such a new software interface, it looks to the host system as though the guest were physically connected to the interface using a network cable: the host can send data to the guest through that interface and receive data from it. This means that you can set up routing or bridging between the guest and the rest of your network.
VirtualBox needs a device driver on your host system. The way
Host Interface Networking works has been completely rewritten with VirtualBox 2.0 and 2.1, depending on the host operating system. From the user perspective, the main difference is that complex configuration is no longer necessary on any of the supported host operating systems.
With the new mechanism, to enable Host Interface Networking, You need to follow this procedure.
1) Go to Applications--->System Tools--->Sun xVM Virtualbox
2) Once virtualbox opens you need to open the Settings dialog of a virtual machine, go to the “Network” page .
3) Select “Host Interface” in the drop down list for the “Attached to” field. Finally, select desired host interface from the list at the bottom of the page, which contains the physical network interfaces of your systems.
4) Once you selected you should see similar to the following screen click on ok
On Linux hosts, functionality is limited when using wireless interfaces for Host Interface Networking. Currently, VirtualBox supports only IPv4 over wireless.For other protocols such as IPv6 and IPX, you must choose a wired interface.Also, setting the MTU to less than 1500 bytes on wired interfaces provided by the sky2 driver on the Marvell Yukon II EC Ultra Ethernet NIC is known to cause packet losses under certain conditions.
If you want to know how to setup host interface networking in VirtualBox 2.0 check here