Which network protocol uses port 21?

The network protocol that uses port 21 is FTP (File Transfer Protocol). FTP is commonly used for transferring files between a client and a server on a computer network. Port 21 is specifically designated for FTP control connections. When a user initiates an FTP connection, the client connects to the server's port 21 to establish communication for sending and receiving files. It is important to note that FTP operates in two modes: active mode and passive mode, each using different ports for data transfer.