virtualbox.org

Edit:

I am wanting to setup up 3 virtual networks.
2 of the virtual networks, I am wanting to set up servers and clients that can communicate within their own networks...standard client/server model.
I then want to set up the 3rd virtual network, to setup a single machine to act as router in between the first 2 virtual networks, so I can practice configuring the router/firewall communications.
I want to run RHEL6 in all networks for clients and servers.

I am not sure how to go about setting up the machines to act as the routers..and then how to setup the virtual box settings to allow communications between the virtual networks, and am looking for assistance

I would using Virtualbox's internal networking. Set virtual network 1's guests on internal network, call it "intnet1". Set virtual network 2's guests on internal network, call it "intnet2". (You just call the network a different name, Virtualbox sets up a different internal network automatically.)

Then the router guest would have two virtual network cards, one set to internal network "intnet1" and the other to internal network "intnet2". The router guest OS would be used to connect the two internal networks together with whatever you can set up. If you want internet on the two networks, use a third virtual network card, set to NAT, and share the internet connection in the router guest with the internal networks.