The answer is Layer 3, the Network layer, where encapsulation at the network layer IP addressing occurs. This is correct because the Internet Protocol (IP) is responsible for adding the source and destination IP addresses to the packet, transforming the segment from the Transport layer into a routable packet. On the CompTIA Network+ N10-009 exam, this concept tests your understanding of the OSI model’s encapsulation process, often appearing in scenario-based questions where you must identify which layer adds logical addressing. A common trap is confusing Layer 2 (which adds MAC addresses) with Layer 3, so remember that IP addresses are logical and used for end-to-end delivery across networks, while MAC addresses are physical and used for local hops. Memory tip: think “IP at Layer 3” — the “P” in IP stands for Protocol, and the “3” reminds you it’s the third layer.
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 technician is explaining the concept of encapsulation to a junior technician. At which OSI layer does a packet get encapsulated with a source and destination IP address?
Why wrong: Layer 2 (Data Link) encapsulates the packet into a frame with MAC addresses, not IP addresses. IP addressing is a Layer 3 function.
B
Layer 3
The network layer (Layer 3) adds the IP header containing source and destination IP addresses. This is where logical addressing occurs, enabling routing across networks.
C
Layer 4
Why wrong: Layer 4 (Transport) encapsulates data into segments or datagrams with port numbers for upper-layer services. IP addressing is not added at this layer.
D
Layer 1
Why wrong: Layer 1 (Physical) deals with the actual transmission of bits over the medium. No headers are added at this layer.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Layer 3
At Layer 3 (the Network layer), the packet is encapsulated with a source and destination IP address. This is defined by the Internet Protocol (IP), which handles logical addressing and routing across networks. The IP header is added to the payload from the upper layers, creating a packet that can be forwarded by 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 2
Why it's wrong here
Layer 2 (Data Link) encapsulates the packet into a frame with MAC addresses, not IP addresses. IP addressing is a Layer 3 function.
✓
Layer 3
Why this is correct
The network layer (Layer 3) adds the IP header containing source and destination IP addresses. This is where logical addressing occurs, enabling routing across networks.
Related concept
Read the scenario before looking for a memorised answer.
✗
Layer 4
Why it's wrong here
Layer 4 (Transport) encapsulates data into segments or datagrams with port numbers for upper-layer services. IP addressing is not added at this layer.
✗
Layer 1
Why it's wrong here
Layer 1 (Physical) deals with the actual transmission of bits over the medium. No headers are added at this layer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse Layer 2 MAC addressing with Layer 3 IP addressing, mistakenly thinking the packet is encapsulated with IP addresses at the Data Link layer, but encapsulation with IP addresses occurs strictly at the Network layer.
Detailed technical explanation
How to think about this question
The IP header includes fields like version (IPv4 or IPv6), TTL, protocol, and checksum, in addition to source and destination IP addresses. In IPv4, the header is typically 20 bytes (without options), and the packet is the PDU at Layer 3. A real-world scenario: when a host sends data to a remote server, the IP address is used by routers to make forwarding decisions based on the routing table, while MAC addresses are only relevant within a single broadcast domain.
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 — At Layer 3 (the Network layer), the packet is encapsulated with a source and destination IP address. This is defined by the Internet Protocol (IP), which handles logical addressing and routing across networks. The IP header is added to the payload from the upper layers, creating a packet that can be forwarded by 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.
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Question Discussion
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