Network+ OSI layers: locate the kind of failure
The OSI model is a way to organize observations. A missing optical signal is a physical-layer issue; Ethernet forwarding uses data-link addresses; IP routing belongs to the network layer; TCP reliability belongs to transport. Real products span layers, so use the model to narrow the question rather than to claim that every device performs only one function.
How to read the scenario
A cable problem will not be fixed by changing DNS. Establish whether each lower dependency works before investigating the application.
Identify the required outcome, the evidence already provided, and the limits on the technician’s action. Compare every option against those details before deciding. Record why the closest alternative does not meet the stated need.
Original practice example
An engineer investigates optical power loss on a fiber link before any frames can be received. At which OSI layer is the fault being examined?
- Layer 1 — Physical
- Layer 4 — Transport
- Layer 3 — Network
- Layer 2 — Data link
Show the answer and explanation
Layer 1 — Physical
The physical layer concerns signaling and transmission media such as optical power on fiber.
Study questions
Which objective does this guide support?
N10-009, objective 1.1. This independent guide supports original practice and does not replace the full official objectives.
What should I watch for in a scenario?
A cable problem will not be fixed by changing DNS. Establish whether each lower dependency works before investigating the application.
Source and version
CompTIA official N10-009 objectives, objective 1.1. Verified November 5, 2026. Use the exam code when choosing study materials; similarly named certifications can have different objectives.