Passive transfer

In passive mode FTP, the client initiates the connection to the server to solve the problem of firewalls. During an FTP connection the client opens two unprivileged ports locally (N> 1024 and N+1). The first port contacts the server on port 21, but instead of giving a port command and allowing the server to connect to its data port again, the client gives the PASV command. As a result, the server opens an unprivileged port (P>1024) and sends the port P command back to the client. The client then initiates the connection from port N+1 to port P on the server to transfer data.