TCP port 20 is used by:

TCP port 20 is used by:

• A. SSH
• B. Telnet
• C. SMTP
• D. FTP - Data

Answer: (D)

TCP port 20 is traditionally the data channel for FTP in active mode. FTP uses two connections: a control channel on port 21 and a data channel on port 20 when data is being transferred. In active FTP, the server opens the data connection from its port 20 to the client’s port, which is why port 20 is associated with FTP data. SSH uses port 22, Telnet uses port 23, and SMTP uses port 25, so those are different services. (In passive FTP, the data port is chosen dynamically, but the standard data channel in active mode uses port 20.)

TCP port 20 is traditionally the data channel for FTP in active mode. FTP uses two connections: a control channel on port 21 and a data channel on port 20 when data is being transferred. In active FTP, the server opens the data connection from its port 20 to the client’s port, which is why port 20 is associated with FTP data. SSH uses port 22, Telnet uses port 23, and SMTP uses port 25, so those are different services. (In passive FTP, the data port is chosen dynamically, but the standard data channel in active mode uses port 20.)