With recent version of VirtualBox you no longer need to create a bridge and tap to enable host networking. In the settings panel of the virtual machine, select the network tab, and make sure that its set on bridged adapter. VirtualBox automatically creates a virtual ethernet adapter, and bridges it to the network of the host machine.
Bridging to the host network is necessary for the virtual machine to get an ip address from the router. Such a configuration is needed to communicate with the machine from the outside world. Because of this feature virtual machines can server website, and other services. I use my virtual machine to host this website, a visual ftp server, a wiki, and an EyesOS server.
There are other ways to forward individual ports from the host system to the guest, but then the ports on the host machine will not be accessible on the host; they will virtually represent the guest OS thus hindering connectivity with the host machine. There may be instances where this is what is wanted and more information, including instruction, can be found on this previous post.
The following is a screenshot of the network tab in VirtualBox. It had a bridge adapter option that will autobridge the connection to an existing ethernet adapter. Bare in mind that this is the screen from a Linux install of VirtualBox and it might be different in Windows. I remember in Windows you might have the option to create a virtual ethernet adapter, and then you can bridge the connection all in VirtualBox. Nothing needs to be done manually anymore, and everything can be done from within the VirtualBox configuration panels.
Here is a screen from Windows Vista. Go to the network tab of the virtual machine settings:
Then select the attach to drop down menu:
Then change the attach to, drop down menu, to bridged adapter: