In TCP/IP networking, which well-known TCP port number is used for the FTP control connection (the command channel between client and server)?

Introduction / Context:
File Transfer Protocol (FTP) is one of the oldest application layer protocols used on TCP/IP networks to transfer files between a client and a server. FTP uses a client server model and relies on two separate TCP connections: a control connection and a data connection. Understanding which port numbers are associated with FTP is a common requirement in Cisco and general networking exams.

Given Data / Assumptions:

We are dealing with standard, unencrypted FTP as defined by classic Internet standards.
FTP uses one TCP connection for control commands and another for the actual data transfer.
The question specifically asks for the port number used for the control connection (also called the command channel).

Concept / Approach:
Standard FTP typically uses TCP port 21 for the control connection, where commands like USER, PASS, LIST and RETR are sent. Data transfer commonly uses TCP port 20 from the server side in active mode, or a dynamically negotiated high port in passive mode. It is important to distinguish FTP from other protocols that also use well known ports, such as SSH on port 22 and SMTP on port 25.

Step-by-Step Solution:
Recall the association: FTP control on TCP 21, FTP data commonly on TCP 20 in active mode. Check the options: port 21 is clearly listed and is the known control port for FTP. Port 20 is related to FTP but is used for data transfer, not the main control channel that carries commands. Port 22 belongs to SSH (Secure Shell), which is used for secure remote login, not FTP. Port 25 is used primarily by SMTP for email transfer and port 23 is used by Telnet, so they are incorrect for FTP control.

Verification / Alternative check:
You can verify this mapping by looking at the list of well known TCP ports: 20 (FTP data), 21 (FTP control), 22 (SSH), 23 (Telnet), 25 (SMTP), 80 (HTTP) and so on. Additionally, firewall and access list configurations frequently reference TCP port 21 when allowing or inspecting FTP control traffic. This consistent usage across documentation and configuration examples confirms that TCP port 21 is the correct answer.

Why Other Options Are Wrong:
Option a (port 20) is closely related to FTP but is not the control channel; it is traditionally used for the data channel in active mode FTP.
Option c (port 22) is the default for SSH, not FTP.
Option d (port 25) is associated with SMTP for email, not file transfer via FTP.
Option e (port 23) is the Telnet port used for remote terminal access, not for FTP.

Common Pitfalls:
A frequent source of confusion is mixing up FTP's two ports and assuming that port 20 is the only port associated with FTP. Another pitfall is conflating FTP with secure file transfer protocols such as SFTP, which actually run over SSH on port 22, or FTPS, which uses SSL/TLS. Remember that this question is about classic FTP control, not secure variants.

Final Answer:
The FTP control (command) connection uses TCP port 21.