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FTP Control
Port 21 is primarily used for FTP (File Transfer Protocol) command and control communications, facilitating file transfer sessions between a client and server. It manages authentication and commands that initiate data transfers on a separate data port..
FTP Overview: The File Transfer Protocol (FTP) is a standard network protocol used for transferring files between a client and server over a TCP/IP network. Port 21 specifically handles the command channel, where session negotiation, user authentication, and instructions like PUT, GET, or LIST are exchanged.
Communication Process: When an FTP connection is established, port 21 manages the control commands which coordinate the transfer process. Actual file data is transmitted over a separate data port, typically port 20 in active mode or a dynamically allocated port in passive mode. The separation ensures command integrity while allowing flexible data conversations.
Operational Modes: FTP operates in two modes: active and passive. In active mode, the server initiates the data connection back to the client, which requires specific firewall configurations. Passive mode, more firewall-friendly, has the client initiating both connections, improving compatibility with NAT environments but potentially exposing dynamic high-number ports.