N10-009 Question 85
Single answerRDP: 3389A systems administrator is trying to remotely manage a Windows server at a branch office from the corporate network. The server is online and responds to ping, but the administrator cannot open a Remote Desktop session. A review of the branch firewall shows that only inbound TCP port 3390 is allowed to the server. No custom RDP listener port has been configured on the Windows server. Which of the following is the MOST likely cause of the issue?
- A
The firewall is allowing the wrong port; standard RDP uses TCP 3389 by default
- B
RDP requires UDP 161 to establish the remote session
- C
The server must have TCP 22 open because RDP tunnels through SSH by default
- D
Remote Desktop connections require ICMP to be disabled on the server
Show answer and explanation
Correct answer: A
Explanation
This scenario tests practical recognition of the default port used by Remote Desktop Protocol. By default, RDP uses TCP 3389, and many Network+ questions expect candidates to identify the service-to-port mapping and apply it to troubleshooting. Because the server responds to ping, Layer 3 connectivity is present. The key issue is that the firewall permits TCP 3390 instead of the expected TCP 3389, and the prompt explicitly states that no custom listener port has been configured. In Microsoft documentation and common administrative practice, Remote Desktop Services uses port 3389 unless intentionally changed. A network professional should verify firewall ACLs, NAT rules, and host-based firewall settings to ensure TCP 3389 is permitted when standard RDP access is required.
- A. Correct.
Correct. Microsoft Remote Desktop Protocol listens on TCP port 3389 by default unless the listener is manually changed in the operating system configuration. If the firewall only permits TCP 3390 and the server is still using its default RDP port, connection attempts will fail even though the host is reachable by ping.
- B. Incorrect.
Incorrect. UDP 161 is used for SNMP, not RDP session establishment. While newer Remote Desktop implementations can also use UDP 3389 for performance improvements, the core and default port association for RDP is TCP 3389, not UDP 161.
- C. Incorrect.
Incorrect. TCP 22 is used for SSH, which is a separate remote administration protocol commonly associated with Linux and network devices. RDP does not tunnel through SSH by default on Windows servers.
- D. Incorrect.
Incorrect. ICMP is commonly used for connectivity testing, such as ping, but it is not a requirement for Remote Desktop operation. In this scenario, the successful ping actually confirms basic IP reachability rather than causing the RDP problem.