I just noticed the following. I use an Ubuntu 13.04 desktop as Virtualbox host. On the host I have two ethernet connections configured. There is only one network card, and the network cable is connected to a working router. I use it to switch configuration because sometimes I use this desktop to login on a second router which has a different network range. I switch cables to the second router, switch the network connection in Ubuntu, and can login.
Today Ubuntu switched the connection without me noticing it. The network cable was still connected to the normal router. I started Virtualbox, started up the browser on the guest OS and could browse the internet. On the host I could not browse the internet. After a while I noticed that the connection had switched. After setting it back to normal everything worked like it should.
I would think that all network traffic is passed through the host OS, so host firewall rules and connection settings would limit the guest, but apparently it isn't so.
Can somebody explain how this works? And does this work for Windows or OSX as well? I can't test that at the moment, but am interested if somebody wants to try.