The answer is Layer 2, the Data Link Layer, because a switch forwards frames using only the destination MAC address without modifying the frame and floods unknown unicast traffic to all ports except the ingress. This behavior relies on a MAC address table, which the switch builds by learning source MAC addresses from incoming frames, enabling it to make forwarding decisions at the hardware level without inspecting IP headers or altering the frame’s content. On the CompTIA Network+ N10-009 exam, this question tests your ability to distinguish Layer 2 switching from Layer 3 routing—a common trap is confusing flooding with routing, but remember that routers modify frames (changing MAC addresses) and use IP addresses, while switches do not. A solid memory tip: “Layer 2 looks at MAC, never looks back” meaning it only cares about the hardware address and never changes the frame.
N10-009 Networking Concepts Practice Question
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.
A network device receives a frame and forwards it based on the destination MAC address. The device does not modify the frame and only floods unknown unicast frames. At which layer of the OSI model does this device operate?
Why wrong: Layer 1 devices like hubs and repeaters do not make forwarding decisions based on MAC addresses. They only regenerate signals on all ports.
B
Layer 2
This describes a switch, which operates at the Data Link layer (Layer 2). It uses MAC addresses to forward frames and floods unknown unicast frames.
C
Layer 3
Why wrong: Layer 3 devices like routers use IP addresses to make forwarding decisions and can modify packets (e.g., TTL). They do not operate primarily on MAC addresses.
D
Layer 4
Why wrong: Layer 4 (Transport layer) handles end-to-end communication and segmentation. Devices at this layer include firewalls that filter based on ports, but they do not forward based on MAC addresses.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Layer 2
This device operates at Layer 2 (Data Link Layer) because it forwards frames based on the destination MAC address, does not modify the frame, and floods unknown unicast frames. These behaviors are characteristic of a transparent bridge or switch, which uses a MAC address table to make forwarding decisions without examining IP addresses or modifying the frame. Layer 2 devices do not perform routing or alter the frame's content, distinguishing them from Layer 3 routers.
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.
✗
Layer 1
Why it's wrong here
Layer 1 devices like hubs and repeaters do not make forwarding decisions based on MAC addresses. They only regenerate signals on all ports.
✓
Layer 2
Why this is correct
This describes a switch, which operates at the Data Link layer (Layer 2). It uses MAC addresses to forward frames and floods unknown unicast frames.
Related concept
Read the scenario before looking for a memorised answer.
✗
Layer 3
Why it's wrong here
Layer 3 devices like routers use IP addresses to make forwarding decisions and can modify packets (e.g., TTL). They do not operate primarily on MAC addresses.
✗
Layer 4
Why it's wrong here
Layer 4 (Transport layer) handles end-to-end communication and segmentation. Devices at this layer include firewalls that filter based on ports, but they do not forward based on MAC addresses.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between Layer 2 switching and Layer 3 routing by describing a device that forwards based on MAC addresses but does not modify frames, leading candidates to mistakenly think of a router (Layer 3) because they associate 'forwarding' with routing, when in fact the key clue is the lack of frame modification and the flooding of unknown unicasts.
Detailed technical explanation
How to think about this question
Under the hood, a Layer 2 switch builds its MAC address table by learning the source MAC address of incoming frames and associating it with the ingress port. When an unknown unicast frame arrives (destination MAC not in the table), the switch floods it out all ports except the receiving port, a process defined in IEEE 802.1D for transparent bridging. In real-world scenarios, this flooding behavior can cause unnecessary traffic in large VLANs, which is why features like MAC address table aging and port security are critical for performance and security.
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 2 — This device operates at Layer 2 (Data Link Layer) because it forwards frames based on the destination MAC address, does not modify the frame, and floods unknown unicast frames. These behaviors are characteristic of a transparent bridge or switch, which uses a MAC address table to make forwarding decisions without examining IP addresses or modifying the frame. Layer 2 devices do not perform routing or alter the frame's content, distinguishing them from Layer 3 routers.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network switch forwards frames based on which address?
easy
✓ A.MAC address
B.IP address
C.Port number
D.Domain name
Why A: A network switch operates at Layer 2 (Data Link Layer) of the OSI model and uses MAC addresses to make forwarding decisions. When a frame arrives, the switch examines the destination MAC address, looks it up in its MAC address table (CAM table), and forwards the frame only to the port associated with that MAC address, reducing collision domains and improving network efficiency.
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This N10-009 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the N10-009 exam.
Question Discussion
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