PASV mode works by allowing FTP clients to initiate sending of both control and data messages. Ordinarily, only FTP servers initiate the data requests. Because many client firewalls reject incoming messages like these FTP requests, PASV mode makes FTP "firewall-friendly."
Most FTP clients, including Web browsers like Internet Explorer, support a PASV FTP option. However, configuring PASV on the client doesn't guarantee that PASV mode will work, as FTP servers may choose to deny PASV mode connections. Some network administrators disable PASV mode on FTP servers because of the additional security risks PASV entails.