This N10-009 practice question tests your understanding of networking concepts. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 engineer is troubleshooting a communication issue between two hosts. The engineer determines that the data is being segmented at the source and reassembled at the destination, but the segments are arriving out of order. Which OSI layer is responsible for sequencing the segments?
Why wrong: Incorrect. The Physical layer deals with the transmission of raw bits over a medium and has no concept of sequencing.
B
B) Data Link layer
Why wrong: Incorrect. The Data Link layer encapsulates packets into frames and handles MAC addressing, but does not manage sequencing of application data.
C
C) Network layer
Why wrong: Incorrect. The Network layer is responsible for logical addressing and routing, not for sequencing or reassembly of segmented data.
D
D) Transport layer
Correct. The Transport layer (e.g., TCP) segments data at the source, assigns sequence numbers, and reassembles the data in correct order at the destination.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
D) Transport layer
The Transport layer (Layer 4) is responsible for end-to-end communication, including segmentation, reassembly, and sequencing of data segments. Protocols like TCP use sequence numbers to order segments correctly at the destination, even if they arrive out of order. This ensures the upper layers receive a complete, ordered data stream.
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.
✗
A) Physical layer
Why it's wrong here
Incorrect. The Physical layer deals with the transmission of raw bits over a medium and has no concept of sequencing.
✗
B) Data Link layer
Why it's wrong here
Incorrect. The Data Link layer encapsulates packets into frames and handles MAC addressing, but does not manage sequencing of application data.
✗
C) Network layer
Why it's wrong here
Incorrect. The Network layer is responsible for logical addressing and routing, not for sequencing or reassembly of segmented data.
✓
D) Transport layer
Why this is correct
Correct. The Transport layer (e.g., TCP) segments data at the source, assigns sequence numbers, and reassembles the data in correct order at the destination.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the misconception that the Network layer (Layer 3) handles sequencing because it deals with packet fragmentation and reassembly, but fragmentation is based on MTU size, not sequence numbers for ordering; sequencing is strictly a Transport layer function.
Detailed technical explanation
How to think about this question
TCP at the Transport layer assigns a sequence number to each byte of data, allowing the receiver to reorder segments using a sliding window and to detect missing segments for retransmission. In contrast, UDP does not provide sequencing, so out-of-order delivery is passed directly to the application. Real-world scenarios like VoIP or video streaming often use UDP and handle reordering at the application layer, while file transfers rely on TCP's sequencing to ensure data integrity.
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: D) Transport layer — The Transport layer (Layer 4) is responsible for end-to-end communication, including segmentation, reassembly, and sequencing of data segments. Protocols like TCP use sequence numbers to order segments correctly at the destination, even if they arrive out of order. This ensures the upper layers receive a complete, ordered data stream.
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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