Reserved TCP port used by File Transfer Protocol (FTP) to initiate and control connections to FTP servers.
To establish an FTP session, clients initiate a connection to an FTP server that listens on TCP port 21 by default. FTP servers respond with messages that prompt the client for FTP login credentials (username and password). FTP servers do not, however, send files from port number 21. Instead, the FTP protocol allows for a second connection to be established for data transfer after the control connection is established. Note that only FTP servers use port 21, not FTP clients.