N10-009 Question 2
Single answer1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.A network administrator is troubleshooting a problem in a warehouse. Several handheld devices can join the corporate Wi-Fi network and receive valid IP addresses from DHCP, but they cannot open the inventory application hosted on a remote server. The administrator successfully pings the default gateway from a handheld device, but a TCP connection test to the application server on port 443 fails. Based on the OSI model, which layer should the administrator investigate first?
- A
Layer 2, because successful Wi-Fi association confirms the issue is with framing and MAC addressing
- B
Layer 3, because DHCP and gateway reachability indicate IP addressing is the only remaining problem
- C
Layer 4, because IP connectivity exists but the session to the application's TCP port cannot be established
- D
Layer 7, because the inventory application is not opening, so the problem must be with the application itself
Show answer and explanation
Correct answer: C
Explanation
This scenario follows a practical OSI-based troubleshooting approach: start with the evidence and identify the highest confirmed working layer. The devices associate to Wi-Fi and obtain DHCP addresses, showing Layers 1 through 3 are at least partially operational. The administrator can also ping the default gateway, further supporting local IP connectivity. However, the failure to establish a TCP connection to port 443 points most directly to Layer 4, where TCP handles connection setup and port-based communication. In real environments, this commonly leads administrators to check host firewalls, network firewalls, ACLs, load balancers, or whether the server process is listening on the expected port. This aligns with standard troubleshooting practices described in CompTIA Network+ objectives and common vendor documentation from Cisco and Microsoft, which emphasize isolating faults by OSI layer and using protocol-specific tests such as ping for Layer 3 reachability and TCP port tests for Layer 4 validation.
- A. Incorrect.
Incorrect. Layer 2 covers local link functions such as framing, MAC addressing, and wireless association. In this scenario, the devices are already joining Wi-Fi and obtaining network access, which suggests Layer 2 is functioning sufficiently for communication on the local segment. A candidate might choose this if they associate Wi-Fi problems with Layer 2 by default, but the evidence points higher in the stack.
- B. Incorrect.
Incorrect. Layer 3 is responsible for logical addressing and routing. The handheld device receives a valid IP address via DHCP and can ping the default gateway, which indicates basic Layer 3 connectivity on the local network is working. While Layer 3 issues beyond the gateway could still exist in some cases, the failed TCP connection to port 443 more directly points to transport-level troubleshooting first.
- C. Correct.
Correct. Layer 4, the Transport layer, is responsible for end-to-end communications using protocols such as TCP and UDP, including port numbers and connection establishment. Because the device has basic network connectivity but cannot establish a TCP connection to the server's HTTPS port, the administrator should first investigate Layer 4 issues such as blocked ports, ACLs, firewall rules, or a service not listening on the expected port.
- D. Incorrect.
Incorrect. Layer 7 involves application services and interfaces, and an application fault is possible in some outages. However, the failed TCP connection test to port 443 indicates the problem occurs before normal application-layer communication can even begin. A candidate might select this because users report that the app will not open, but the test results narrow the issue to transport connectivity first.