I can advise you to read the tutorial that @Ctek has written: How-To: Create a NethServer instance in VirtualBox machine
Then RED interface is bridged and the GREEN interface is set as host only. The host only settings are done in a way you can reach the GREEN interface from the host.
I have used this setup several times. It has great advantages since you can actually have a full NS instance reachable from the GREEN interface of the VM without interfering in the LAN subnet.