N10-009 Question 1
Single answer1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.A network administrator is troubleshooting a problem in a small office after replacing an unmanaged switch with a managed switch. Users report that they can access printers and file shares within the local VLAN, but they cannot reach websites by name. When the administrator tests from a workstation, pinging the default gateway by IP address succeeds, and pinging a public IP address also succeeds. However, pinging a website FQDN fails. Based on the OSI model, at which layer is the problem most likely occurring?
- A
Layer 3 - Network
- B
Layer 4 - Transport
- C
Layer 7 - Application
- D
Layer 1 - Physical
Show answer and explanation
Correct answer: C
Explanation
This scenario is a classic example of using the OSI model to isolate a fault domain. Because the client can reach the default gateway and a public IP address, Layers 1 through 3 are demonstrably operational. The inability to reach a website by fully qualified domain name points to DNS resolution, which Network+ commonly associates with the Application layer (Layer 7) when mapping services to the OSI model. In practice, DNS uses UDP port 53 and sometimes TCP port 53, but exam objectives typically emphasize the service role rather than only the transport protocol. This layered troubleshooting approach aligns with standard networking best practices: verify physical connectivity first, then local IP configuration, then routed connectivity, and finally higher-layer services such as DNS.
- A. Incorrect.
Incorrect. Layer 3 is responsible for logical addressing and routing, such as IP and default gateway communication. Because the workstation can successfully ping both its default gateway and a public IP address, Layer 3 connectivity is functioning. A candidate might choose this option because routing issues are common after switch changes, but the successful IP-based tests indicate the network layer is not the primary problem.
- B. Incorrect.
Incorrect. Layer 4 handles end-to-end transport services such as TCP and UDP. While DNS commonly uses UDP and sometimes TCP, the scenario specifically points to successful connectivity by IP address but failure when using hostnames. That indicates name resolution rather than a general transport-layer failure. Someone might select this because DNS traffic uses transport-layer protocols, but the service issue itself is typically classified higher in the OSI model on Network+ exams.
- C. Correct.
Correct. Layer 7 includes application services such as DNS, which translates hostnames to IP addresses. The workstation can communicate locally and externally by IP, so basic physical, data link, network, and likely transport connectivity are working. The failure only occurs when using an FQDN, which strongly indicates a DNS/name-resolution issue at the application layer.
- D. Incorrect.
Incorrect. Layer 1 involves the physical medium, signaling, cabling, and link lights. If Layer 1 were the issue, the workstation would not be expected to reach the default gateway or a public IP address successfully. A user might pick this option because the problem started after a hardware replacement, but the evidence shows physical connectivity is intact.