What is the network port number commonly used for FTP?

The network port number commonly used for FTP, or File Transfer Protocol, is 21. FTP is a standard network protocol used for transferring files between a client and server over a TCP/IP connection. Port 21 is designated for command control, where the client sends commands to the FTP server.

When establishing an FTP connection, the initial communication happens over port 21 to set up the session and handle login credentials. After this setup, the actual data transfer may occur over a separate data connection, which can use a different port, depending on whether it's in active or passive mode. This is critical for understanding how FTP operates in different network environments.

The other options listed are associated with different protocols — port 80 is used for HTTP traffic, port 443 is for HTTPS (secure HTTP), and port 25 is typically designated for SMTP (Simple Mail Transfer Protocol), which is used for sending emails. Each of these protocols serves a distinct purpose in networking, which emphasizes the importance of recognizing the correct port number for FTP in the context of file transfers.