First thing: Define a range of PASV TCP ports on your router or firewall. From my days of running WS-FTP server, they used ports 4900 - 4910 by default. I got hooked and use these ones. The ports can be anything. If you are not sure, check with your network admin.
Secondly - Open IIS Manager, select the computer name, open FTP Firewall Support
Next, type in the Data Channel Port Range and external IP address. The External IP address can be left blank and filled in at a site level. The port range will be inherited by each site. Assuming you have static IP address on each site, there won't be an issue sharing this range across multiple FTP sites.
Nguồn (đầy đủ link tham khảo):
The default dynamic port range for TCP/IP has changed in Windows Vista and in Windows Server 2008
Configuring FTP Firewall Settings in IIS 7