In Feburary 2016 17,
Go to the configuration page for your router, which is often online and available via the router manufacturer's website.
Access the page for the router's port range forwarding. The link or tab for applications is usually the place to go.
Input 'FTP' in the first available box under the 'Application' column. In the row for this new application, put '21' underneath both 'Start' and 'End.'
Select 'TCP' in the drop-down box under 'Protocol' for the same row with your inputted FTP application. Under 'IP Adress,' input the address for the machine with your FTP server.
Create a second application row under the FTP row, inputting 'PSAV' in the Application box. Input the port range for your FTP server in the Start and End boxes. Input 'TCP' and the same IP address for the FTP server.
In Feburary 2016 17,