How do I configure my router (NETGEAR FR114P) so people can connect using passive mode?

I have set up an FTP server, but am told I have to open ports 50000 - 51000 on my router to enable Passive Mode connections. I can presently connect with Active Mode. The problem is that when I try to open up ports 50000 - 51000, I get the message "Virtual Server Exists", and it won't allow me to do it.

How do I get around this?

How do I enable people to connect to my server using passive mode?

With a router, I have read that you need to set a passive port range.

Below are a couple of the many sites that I have visited that says to set a passive port range with a router.

http://users.pandora.be/UyTiEmAn/website/tutorial1.htm

http://pctechforums.com/modules.php?name=FTP

I am just wondering if the problem lies with my client being behind a router / firewall. I have connected to other FTP sites before without having to open any ports on my router / firewall, but it has got me thinking. Should I be opening any inbound or outbound ports on my client to connect to FTPs in passive mode?

UPDATE:

Thanks for all who helped. I found the problem why I could open ports 50000 - 51000. It was a bug in the router / firewall firmware. I updated the firmware and everything is OK.