I want to create a private network between virtual machines running in multiple hosts using Virtualbox. Linux will be used as guest operating system, and Windows in the hosts.
How do I set this up using Virtualbox?
Option 1: Hardware
This should give you a private network between hosts
Option 2: Software
By connecting the machines using virtual private networking, you should be able to create a private LAN that only those machines exist on.
Logmein Hamachi is a free VPN solution which supports Windows, Linux and OS X. Quite easy to install. It may be a little "overpowered" for your situation but should work.
Edit: Another, similar option is wippien.com, look at their "Mini Wippien" option. Quote: "Unlike Wippien that combines two worlds together, miniVPN is small appliation that will give you benefit of having VPN without instant messaging capabilities."
If the host machines are all on the same network, you may be able to configure DHCP to assign the MAC addresses used by VirtualBox (or VMWare) VMs to a different 'virtual' network.
For example, say all your host machines run on 192.168.0.0. You could configure DHCP to assign all computers with MAC addresses beginning with 80:00:27 to the 192.168.99.0 network.
Hope this helps.