My Guest: Windows XP
When I connect to my ADSL connection on my linux, the guest (Win XP) can use the internet. I mean I can ping google.com over there. On the Windows XP (the guest) I can ping google, 10.0.2.15 (loop back) and 10.0.2.2 (the linux host) but on the other hand on my linux I can ping none of them.
By the way, I've installed Guest Additions too.