Which protocol uses port 21?

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Multiple Choice

Which protocol uses port 21?

Explanation:
Port numbers map to specific services, and port 21 is reserved for the FTP control channel. When an FTP session starts, the client connects to the server on port 21 to send commands (like login, change directory, list files) and receive responses. Data transfers happen over a separate data connection, which works differently depending on the mode: in active mode the server initiates the data connection from port 20, while in passive mode the server picks a high, dynamic port and the client connects to it. That combination is why FTP is associated with port 21. Other options use different ports: SMTP typically uses 25 (or 587/465 for submission), HTTPS uses 443, and ARP operates at a layer that doesn’t use TCP/UDP ports.

Port numbers map to specific services, and port 21 is reserved for the FTP control channel. When an FTP session starts, the client connects to the server on port 21 to send commands (like login, change directory, list files) and receive responses. Data transfers happen over a separate data connection, which works differently depending on the mode: in active mode the server initiates the data connection from port 20, while in passive mode the server picks a high, dynamic port and the client connects to it. That combination is why FTP is associated with port 21. Other options use different ports: SMTP typically uses 25 (or 587/465 for submission), HTTPS uses 443, and ARP operates at a layer that doesn’t use TCP/UDP ports.

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