people use FTP to copy a file from pc to another pc over a tcp -based network,just like internet. FTP services are generally run on two ports 20 and 21.
Port 20 is used between the client and the server data flow, and port 21 for the transmission of control flow, and is leading to ftp server Import command.
FTP has two modes: active and passive.
Active mode requires both client and server-side open and listen on a port to create a connection.
Client will have some problem when install a firewall.but Passive mode can avoid this problem.It only requires the server to generate a process of monitoring the appropriate port.
link http://en.wikipedia.org/wiki/File_Transfer_Protocol

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