Filezilla unroutable address

Recently I encountered a problem while connecting an ftp server running behind a router, which was behind a modem (typical setup now-a-days). Connecting to the server internally from the network works fine (including router and modem) however from the external WAN address yields something like this...

Command: PWD
Response: 257 "/" is current directory.
Command: TYPE I
Response: 200 Type set to I
Command: PASV
Response: 227 Entering Passive Mode (172,168,1,11,8,33)
Status: Server sent passive reply with unroutable address. Using server address instead.
Command: LIST
Response: 425 Can't open data connection.

The problem is where the Passive Mode is not opened and is stuck. Using command line ftp, I found the same problem however the ftp utility went on with certain other combination of the command and did manage to get the directory listing. something like this...
229 Entering Passive Mode (|||49905|)
ftp: Can't connect to `100.100.1001.11': Operation timed out
200 Command EPRT okay.
150 File status okay; about to open data connection.
drwx------   3 user group            0 Jan 23 13:43 Photos
-rw-------   1 user group           28 Sep 29 04:35 README.txt
226 Closing data connection.

To the solve the problem
Go in filezilla-preferences or Edit-Options and selecting the 'Passive Mode' option and change the selection to enter in active mode when passive mode fails. I am not sure about the implications of the solution, for me it was a quick workaround.