Explicit FTP over TLS
In Explicit FTP over TLS, the FTP Client sends a specific command AUTH TLS to the FTP Server to establish the secure TLS Connection. The default FTP Control channel port 21 is used for secure TLS communication. Initially FTP Client connects to the server. and the FTP Client explicitly requests TLS encryption to turn on. User credentials are sent securely encrypted over the network for authentication in explicit FTP over TLS.
Implicit FTP over TLS
In Implicit FTP over TLS, there is no need for the FTP Client to send any command to turn on TLS security. In implicit FTP over TLS, TLS handshake must be negotiated before any FTP commands can be sent by the FTP client to the FTP Server. Encryption is automatically turned on after the FTP client connects to the FTP server. TCP port 990 to be used for encrypted connections. User credentials are sent securely encrypted over the network for authentication in implicit FTP over TLS also.
Following image describes the difference between explicit FTP over TLS and implicit FTP over TLS