FTP is an unusual service in that it utilizes two ports, a 'data' port and a 'command' port (also known as the control port). Traditionally these are port 21 for the command port and port 20 for the data port.

Which 2 ports are used for FTP?

The Protocol

Port numbers 21 and 20 are used for FTP. Port 21 is used to establish the connection between the 2 computers (or hosts) and port 20 to transfer data (via the Data channel).

Why are there two ports for FTP?

FTP is an old protocol. That's really the only reason. The designers thought that the amount of data flowing over the data port would make it so that they couldn't send control commands in a timely manner, so they did it as two ports.