In addidtion to NATing, you can also have access lists on routers which polices traffic in a similiar way to a firewall.

If anyone wants to know more about routers ie. How to configure the interfaces, adding routes, creating access lists, passwords etc.. You can download a product called "Boson Router Simulator" which is a software application that you can practise all these important things on, without the worries of screwing up any actual routers.

I found this software extremely helpful, hope that everyone else does as well.

Go to:

www.routersimulator.com/

to get the demo.