VirtualBox guest network lost after host disconnects

Solution 1:

NAT on VirtualBox is not that good you might want to try Host-only with some "host natting" as explained in this article : http://akutz.wordpress.com/2009/08/20/building-a-better-os-x-firewall-or-how-i-solved-the-nat-problem-for-virtualbox/

Solution 2:

I operate different virtual servers here and had similar problems (and more) when using NAT. What works best for me is using bridged conenctions: Create a network bridge on the host OS (basically not bridging anything but the primary network connection) and use bridged networking in VirtualBox using the newly created bridge. My hosts are Windows Vista and Windows 7, Clients Windows Server 2008 and debian.

This will put your clients in the same net as your host and therefore make host and client see each other as well as different clients see each other (if you have mutliple clients on the same host).

I don't know if there are any side effects and do have no clue about the technical background of bridging, however, so take this advice with special caution.