Port 21
Port number 21 is a reserved port in TCP/IP networking, meaning that it is listed with IANA. Port 21 is utilized by the File Transfer Protocol for FTP control traffic.
FTP clients initiate an FTP session by connecting to port number 21 on the FTP server. An FTP server responds from port 21 with messages that prompt the client for an FTP session login (username and password). Note that FTP clients do not use port 21, only the server.
FTP servers also do not send files from port number 21. Instead, the FTP protocol allows for a second connection to be established for data transfer. Port 21 represents the server-side of a "control" or "command" connection whereas the alternate server port represents the "data" connection.
» Back to the Network Ports Glossary
