How do you change default ftp data port?

I have a specific off-site portion of my network highly secured. I've only let a few things through and two of those are the ftp site (the ftp(21) and ftp-data(20) tcp) ports. I've recently been asked to create an anonymous ftp site so that we can post videos that anyone can get to. I don't want to go through the hassle of obtaining another external IP so I figured I would just change the port on the new ftp site (port tcp/12121 for example). This seems to have worked internally but I get an error when outside of the firewall. I assume this is because the anon ftp site is trying to go outbound on a random port where normally it would use port 20 (ftp-data). I don't want to open up extra ports (even if they are outbound) so I'm trying to figure out what port its trying to use. Nothing is showing up in my firewall log which makes me think its not being blocked but I can't come up with any other answer. Anyone have any ideas on either how to change the ftp-data port or what might be causing the "PORT" errors when accessing the site externally?

Thanks!

 

First make sure that you open the new ftp ports in the Firewall pointing toward the IP of the computer with the FTP server.

To access the ftp from the out side.

If people type your IP address they should type (an example use the numbers that pertaining to you) 65.12.2.33:2121 I.e. the IP colon and the ftp port number.

If you use a Dynamic IP Service then you can redirect the ports at the service.