Today I was setting up some test machines running a minimal debian installation.
Since it's not that handy to work always in the virtualbox window, I tried to connect to them by using ssh.
In the debian machine I typed:
$ su $ apt-get install openssh-server
And tried to connect to the machine. It did not work.
I found plenty instructions how to fix ports and stuff with host-only-networks, but figured out, this is not what I was searching for!
The reason was, that virtualbox's default network mode is NAT.
I changed it to "network bridge". After that a restart of the machine did the trick, my router gave a proper IP to the guest debian.