A client packet is received by a server with a destination port number of 21. What service is being requested?

A client packet is received by a server with a destination port number of 21. What service is being requested?

• A. HTTP
• B. FTP
• C. SMTP
• D. DNS

Answer: (B)

A destination port number of 21 indicates that the service being requested is the File Transfer Protocol (FTP). FTP utilizes port 21 for its control commands. When a client sends a packet to a server with this port number, it signifies that the client is looking to initiate a connection to upload or download files, manage file permissions, or perform other file operations associated with FTP. In contrast, HTTP operates on port 80, SMTP functions on ports 25 (or 587 for submissions), and DNS primarily uses port 53. Knowing the specific port numbers associated with different services is essential for understanding network communication and ensuring that the correct protocols are invoked for specific tasks. Thus, in this scenario, the server receiving a packet on port 21 is indicative of an FTP request.

A destination port number of 21 indicates that the service being requested is the File Transfer Protocol (FTP). FTP utilizes port 21 for its control commands. When a client sends a packet to a server with this port number, it signifies that the client is looking to initiate a connection to upload or download files, manage file permissions, or perform other file operations associated with FTP.

In contrast, HTTP operates on port 80, SMTP functions on ports 25 (or 587 for submissions), and DNS primarily uses port 53. Knowing the specific port numbers associated with different services is essential for understanding network communication and ensuring that the correct protocols are invoked for specific tasks. Thus, in this scenario, the server receiving a packet on port 21 is indicative of an FTP request.