[SOLVED] Create virtual network with virtualbox

Hi,

This acutally is a virtualbox-related question so I hope I'll get help here anyway.
I am currently trying to create a test-setup for my firewall.
For this I got 4 virtual boxes running ubuntu-server. Let's call them A, B, C and D.
Here a quick view if it helps:
A 192.168.1.1
B 192.168.5.235
C 10.65.66.130
D with 3 interfaces:
eth0 192.168.1.254
eth1 10.65.66.131
eth2 10.65.66.129
All 10.65.66.xx-adresses are actually static IPs accessible from the internet.

My issue right now is that I'm not sure how to link those virtual machines.
I want to create a network where only these 4 machines and maybe a fifth for testing have access to (no access to the host-network).

Any ideas?