This N10-009 practice question tests your understanding of networking concepts. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
At which OSI layer does a router primarily operate to make forwarding decisions based on IP addresses?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
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Layer 3 (Network)
A router primarily operates at Layer 3 (Network) of the OSI model because it uses logical IP addresses (e.g., IPv4 or IPv6) to make forwarding decisions. The router examines the destination IP address in the packet header, performs a longest-prefix match against its routing table, and determines the next-hop interface. This layer is responsible for end-to-end delivery and path selection across multiple networks.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Layer 1 (Physical)
Why it's wrong here
Layer 1 deals with physical transmission of bits; routers use higher-layer addressing.
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Layer 2 (Data Link)
Why it's wrong here
Switches operate at Layer 2 using MAC addresses; routers use IP addresses.
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Layer 3 (Network)
Why this is correct
The Network layer handles logical addressing (IP) and routing decisions.
Related concept
Read the scenario before looking for a memorised answer.
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Layer 4 (Transport)
Why it's wrong here
The Transport layer manages end-to-end connections (TCP/UDP); routers make decisions based on IP, not ports.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the misconception that routers operate at Layer 2 because they forward frames, but the key distinction is that routers make forwarding decisions based on Layer 3 IP addresses, not Layer 2 MAC addresses.
Detailed technical explanation
How to think about this question
Routers maintain a routing table populated via static routes or dynamic routing protocols (e.g., OSPF, EIGRP, BGP). When a packet arrives, the router strips the Layer 2 frame, inspects the Layer 3 header, and performs a route lookup using the destination IP address; if no match is found, the packet is dropped (unless a default route exists). In IPv4, the Time-to-Live (TTL) field is decremented at each hop to prevent routing loops, and if TTL reaches zero, the router sends an ICMP Time Exceeded message back to the source.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Networking Concepts — This question tests Networking Concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Layer 3 (Network) — A router primarily operates at Layer 3 (Network) of the OSI model because it uses logical IP addresses (e.g., IPv4 or IPv6) to make forwarding decisions. The router examines the destination IP address in the packet header, performs a longest-prefix match against its routing table, and determines the next-hop interface. This layer is responsible for end-to-end delivery and path selection across multiple networks.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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