Filezilla FTP Server Ports - Active Connections

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http://serverfault.com – I have been obtaining errors like below because I did not specify enough ports for the active FTP connections. Response: 150 Opening data channel for directory list. Response: 425 Can't open data connection.Error: Failed to retrieve directory listing Things seem to work nicely with limited ports, but when I perform actions that cause very rapid short-lived connections, something like 20-30% of the connections drop with the error above. I started with ports 50000-50100. When I opened up to 50000-52000, the errors disappeared. Why did this fix my problem? I would like to understand why (HowTos)