Active FTP vs Passive FTP

Active FTP :   
     command : client >1024 -> server 21   
     data    : client >1024 <- server 20   
 

Passive FTP :   
     command : client >1024 -> server 21   
     data    : client >1024 -> server >1024

FTP operates on the application layer of the OSI model, and is used to transfer files using TCP/IP.^[3] To do so, an FTP server has to be running and waiting for incoming requests.^[3] The client computer is then able to communicate with the server on port 21.^[3][4] This connection, called the control connection,^[5] remains open for the duration of the session. A second connection, called the data connection,^[2][5] can either be opened by the server from its port 20 to a negotiated client port (active mode), or by the client from an arbitrary port to a negotiated server port (passive mode) as required to transfer file data.^[2][4] The control connection is used for session administration, for example commands, identification and passwords exchanged between the client and the server using a telnet-like protocol.^[6] For example "RETR filename" would transfer the specified file from the server to the client. Due to this two-port structure, FTP is considered an out-of-band protocol, as opposed to an in-band protocol such as HTTP.^[6]

Ref:ftp modes
wiki FTP
FTP session sequence