Stephen, the long answer to why Private Host is to have similar environments with VirtualBox. As it turns out VirtualBox NAT interfaces do not allow communication with the host, unlike VMware which does.
That said, if you’re getting an IP address on the screen it’s strange that you would have problems reaching it, do you have any special firewall software running?
Is it possible for you to post the following:
The IP address your Sandbox reports
Open a Windows command prompt and run “netstat -nr” and post those results?