FTP (File Transfer Protocol) is one of the protocols in the TCP/IP protocol group. By default, FTP protocol uses two TCP ports, 20 and 2 1, where 20 is used to transmit data and 2 1 is used to transmit control information. However, whether to use 20 as the data transmission port is related to the transmission method used by FTP. If active mode is adopted, the data transmission port is 20; > if passive mode is adopted; The high random port of 1024 should be decided by the server and the client through negotiation.

The control connection is established before the data connection, so the port number used by FTP file transfer to establish the connection is port 2 1.