FTP to NAS from outside LAN

Based on the following article, I purchased a 1 TB LaCie ethernet disk to use as a NAS (network attached storage):
http://www.macworld.com/article/136632/2008/11/networkattachedstorage.html

I have a NAS set up and I can access it within my LAN via FTP and AFP.

Can I FTP to and from this drive when I am on the road via the Internet?

I have tried using my IP address of the drive and my FTP programs cannot find it when I am not connected to my LAN. I set the firewall of the DSL modem/router to allow FTP traffic (forwarding ports 20 & 21) and still no luck. Do I need to use the IP address of my DSL modem which is very different from the IP address of the network drive?

There are setting on the modem that I do not understand, such as public vs. private LAN. Right now, I have only a private LAN.

Suggestions?

TEG said:

First, you need to know the IP address your modem is connecting to the Internet on. Then you need a router setup that will route messages sent to the modem from the outside on a particular port to a particular internal IP address inside, like your NAS.

TEG

Click to expand...

So how do I find out what the IP address is of my modem and if it is dynamically assigned?

I think I assigned the FTP to go to ports 20 & 21 to the NAS.