Active ftp mode

Client starts connection with server on port TCP 21

Servers tells client to start listning on a port higher then TCP 1024

Server starts sending data traffic from TCP 20 to the TCP 1024 or higher port

EG > the server connects to the client on a dynamic port

Passive ftp mode

Client starts connection with server on port TCP 21

Server tells the client that he is going to open a dynamic port (TCP 1024 or higher) and

The client has to connect to that port.

EG > the client connects to the server on a dynamic port

Port 20 is NOT used in passive ftp mode.