What Are FTP Working Modes and What Are Their Differences

There are two FTP working modes: port (active) mode and passive (PASV) mode. These two modes are significant only to the FTP server.
Port (active) mode
The client sends a request to the FTP port (port 21 by default) to negotiate an FTP control connection. After the FTP server honors the request, the control connection is created. To transmit data, the client must open a port for receiving data and send a message over the control connection to the server to inform the server of the opened port. After receiving the message, the server sends a request from port 20 to the client port specified in the message to create a data channel for data transmission.
Passive mode
The client sends a request to the FTP port (port 21 by default) to negotiate an FTP control connection. After the FTP server honors the request, the control connection is created. To transmit data, the server must use the PASV command to send a message over the control connection to the client to inform the client of the opened port. After receiving the message, the client sends a request to the server port specified in the message to create a data channel for data transmission.