N10-009 Question 55
Single answerTelnet: 23A network administrator is troubleshooting remote access to a legacy router in a lab environment. The administrator can ping the router successfully, but when attempting to remotely manage it from a workstation, the connection fails. A packet capture shows the workstation sending traffic to TCP port 23 on the router, but no management session is established. Which of the following is the MOST likely explanation?
- A
The router is not configured to allow Telnet connections on its virtual terminal lines
- B
The workstation should be connecting to UDP port 23 instead of TCP port 23
- C
The router requires HTTPS on port 443 because Telnet uses a web-based management interface
- D
The administrator should use port 22 because Telnet and SSH both use the same default port
Show answer and explanation
Correct answer: A
Explanation
This question tests practical troubleshooting of Telnet connectivity rather than simple recall of a port number. Telnet is a remote terminal protocol that uses TCP port 23. If ICMP ping succeeds, Layer 3 connectivity exists, but application access can still fail if the target service is disabled, filtered, or not configured to accept sessions. In real environments, many routers and switches require explicit configuration of VTY lines for Telnet access, and modern best practice is to disable Telnet entirely in favor of SSH because Telnet sends credentials and session data in cleartext. This aligns with common vendor guidance and security best practices, including recommendations from network equipment manufacturers and security standards bodies to prefer encrypted management protocols such as SSH over Telnet whenever possible.
- A. Correct.
Correct. Telnet uses TCP port 23, so the packet capture showing traffic to TCP 23 is consistent with a Telnet attempt. If the device is reachable by ping but a Telnet management session cannot be established, a common real-world cause is that Telnet access is disabled or not permitted on the router's virtual terminal (VTY) lines. On many network devices, remote access must be explicitly enabled and configured.
- B. Incorrect.
Incorrect. Telnet uses TCP, not UDP. Choosing UDP 23 reflects a protocol confusion. If a candidate knows only the port number but not the transport protocol, this distractor can seem plausible, but it is factually wrong.
- C. Incorrect.
Incorrect. HTTPS uses TCP port 443 and is for secure web management interfaces, not Telnet. Telnet is a separate remote terminal protocol and does not rely on a browser-based interface. This option reflects confusion between command-line remote management and web administration.
- D. Incorrect.
Incorrect. SSH uses TCP port 22, while Telnet uses TCP port 23. They are both remote access protocols, but they do not share the same default port. This distractor targets the common misconception of mixing up Telnet and SSH because they serve similar purposes.