FTP server behind airport extreme

Hey, So I've had a mac mini running as a server for a while now. I did this by using the built in option under the sharing tab. Everything worked like a charm until recently when I purchased my airport extreme. I know it has something to do with the firewall built in and the NAT, which gave it an internal address. So, I tried port forwarding in the airport extreme settings with little luck. I also told the airport to distribute a range of ip addresses to the computers on the network, this works however limits my network to the speed of the internet connection. ( when transferring files between computers, its seems to transfer across the internet rather than between the internal network). If you guys have any input please please help! I've only owned a mac for about 6 months now, love it but need to get past this one issue. thanks