The computer in the internal network has an IP. and the outside computer has and IP. you can connect to the internal comptuer using the internal IP. the internal computer has 2 ip's. it has one if you do start/run/cmd/ipconfig and it has another one when your on the outside. if you want to see. if someone you know has and website. get there IP. and from an outside computer ping your friends website and you will have two diffrent ip's... so do that then you'll see why you cant connect to the computer in the internal network
tae