The answer is Layer 2, the Data Link layer, because this is where a network device encapsulates data into frames and adds source and destination MAC addresses for node-to-node delivery. The Data Link layer takes packets from the Network layer and wraps them in a frame header and trailer, with the MAC addresses ensuring the frame reaches the correct device on the same local network segment, as defined by IEEE 802 standards like Ethernet. On the CompTIA Network+ N10-009 exam, this concept tests your understanding of how frames differ from packets and why MAC addresses are only relevant within a broadcast domain—a common trap is confusing MAC addresses with IP addresses, which operate at Layer 3. To remember, think of the Data Link layer as the "local delivery" layer: it uses MAC addresses like a street address for a house on your block, while IP addresses are like a city-wide zip code. A handy mnemonic is "MAC at Layer 2, IP at Layer 3."
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.
At which layer of the OSI model does a network device encapsulate data into frames and add source and destination 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 – Data Link
The Data Link layer (Layer 2) is responsible for node-to-node communication and encapsulates packets from the Network layer into frames. It adds a header containing the source and destination MAC addresses, which are used for delivery within the same local network segment. This process is defined by IEEE 802 standards such as Ethernet (802.3) and Wi-Fi (802.11).
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
The Physical layer transmits raw bits over the medium but does not create frames or add MAC addresses.
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Layer 2 – Data Link
Why this is correct
The Data Link layer creates frames, adds MAC addresses, and provides error detection. It is the correct layer for this function.
Related concept
Read the scenario before looking for a memorised answer.
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Layer 3 – Network
Why it's wrong here
The Network layer handles logical addressing (IP) and routing decisions, not MAC addressing.
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Layer 4 – Transport
Why it's wrong here
The Transport layer segments data and provides reliable delivery, but it does not use MAC addresses.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between MAC addresses (Layer 2) and IP addresses (Layer 3), and the trap here is that candidates may confuse the encapsulation process and incorrectly associate MAC addressing with the Network layer due to familiarity with IP addressing.
Detailed technical explanation
How to think about this question
Framing at Layer 2 includes not only MAC addresses but also a preamble, start frame delimiter (SFD), and a frame check sequence (FCS) for error detection. In Ethernet, the minimum frame size is 64 bytes (including the FCS) to ensure proper collision detection in half-duplex environments. A real-world scenario where this matters is when a switch learns MAC addresses by examining the source MAC in incoming frames to build its MAC address table, which is critical for efficient forwarding.
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 – Data Link — The Data Link layer (Layer 2) is responsible for node-to-node communication and encapsulates packets from the Network layer into frames. It adds a header containing the source and destination MAC addresses, which are used for delivery within the same local network segment. This process is defined by IEEE 802 standards such as Ethernet (802.3) and Wi-Fi (802.11).
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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