Blocking port on only one network ip connection

I have an NT Workstation running with 2 NICs on it, and it provides FTP service to an internal workgroup, but the external NIC does not need ftp open, in fact, I would like to block ftp on that network connection, but still allow it for the internal one. Thus, I cant simply turn off the ftp publishing service. What is the best way to block ftp on only one network connection?
Thanks!

 

In IIS Admin, does the option to only assign a specific IP address (instead of "All Unassigned ") to the FTP site work? (For IIS5 you also had to disable socket pooling)