Question 23 of 464
N10-009 Networking Concepts Practice Question
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?
⚠ Common exam trap
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.
Answer choices
Why each option matters
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.
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
The Physical layer is concerned solely with the electrical, optical, or radio signals that represent raw bits, along with the physical characteristics of the transmission medium and connectors. It transmits these bits without any understanding of their meaning, grouping, or order within a larger data stream. This layer lacks the intelligence to perform sequencing or reassembly, as its function is simply to move raw data across a physical link.
When this WOULD be correct
A question asking which layer defines the electrical, mechanical, and procedural specifications for activating, maintaining, and deactivating the physical link between end systems.
- ✗
B) Data Link layer
Why it's wrong here
The Data Link layer manages the reliable transfer of data frames between two directly connected nodes, primarily handling physical addressing (MAC addresses) and error detection within a single hop. While it encapsulates network layer packets into frames and ensures their integrity over a local link, it does not maintain an end-to-end sequence for an entire application data stream across multiple network segments. Its scope is local frame delivery, not global data ordering.
When this WOULD be correct
A question about which layer adds a header with MAC addresses for local delivery or performs error detection on a frame would make the Data Link layer correct.
- ✗
C) Network layer
Why it's wrong here
The Network layer's primary role involves logical addressing (IP addresses) and routing packets across different, potentially disparate, networks to reach their ultimate destination. Each packet is treated as an independent unit, and this layer does not inherently guarantee in-order delivery or manage the reassembly of a larger data stream from multiple packets. Its focus is on finding the best path for individual packets, not on the overall sequencing of application data.
When this WOULD be correct
A question asking which layer is responsible for routing packets between different networks, such as 'Which OSI layer uses IP addresses to determine the best path for data?' would have the Network layer as the correct answer.
- ✓
D) Transport layer
Why this is correct
The Transport layer, notably through protocols like TCP, is directly responsible for segmenting application data into smaller units, assigning unique sequence numbers to each segment. This crucial sequencing allows the receiving host to correctly reassemble the data stream in its original order, even if segments arrive out of sequence due to network conditions. It also facilitates reliable delivery, acknowledging received segments and requesting retransmission of any missing ones, ensuring data integrity from source to destination.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓D) Transport layerCorrect answer▾
Why this is correct
The Transport layer, notably through protocols like TCP, is directly responsible for segmenting application data into smaller units, assigning unique sequence numbers to each segment. This crucial sequencing allows the receiving host to correctly reassemble the data stream in its original order, even if segments arrive out of sequence due to network conditions. It also facilitates reliable delivery, acknowledging received segments and requesting retransmission of any missing ones, ensuring data integrity from source to destination.
✗A) Physical layerWrong answer — click to see why▾
Why this is wrong here
The Physical layer (Layer 1) handles raw bit transmission over a physical medium and does not perform any sequencing or reassembly of data segments.
★ When this WOULD be the correct answer
A question asking which layer defines the electrical, mechanical, and procedural specifications for activating, maintaining, and deactivating the physical link between end systems.
Why candidates choose this
Candidates may confuse the physical transmission of bits with the process of data ordering, or they might think that physical layer issues (like signal interference) could cause out-of-order delivery.
✗B) Data Link layerWrong answer — click to see why▾
Why this is wrong here
The Data Link layer handles node-to-node delivery and framing, not end-to-end segment sequencing. Sequencing of segments is a Transport layer function (e.g., TCP sequence numbers).
★ When this WOULD be the correct answer
A question about which layer adds a header with MAC addresses for local delivery or performs error detection on a frame would make the Data Link layer correct.
Why candidates choose this
Candidates may confuse the Data Link layer's role in ordering frames (e.g., in some protocols like HDLC) with the Transport layer's segment sequencing, or think that any 'sequencing' must happen at a lower layer.
✗C) Network layerWrong answer — click to see why▾
Why this is wrong here
The Network layer (Layer 3) handles routing and logical addressing, but not segmentation or sequencing of data segments. Sequencing is a function of the Transport layer (Layer 4), which manages end-to-end communication and reassembly.
★ When this WOULD be the correct answer
A question asking which layer is responsible for routing packets between different networks, such as 'Which OSI layer uses IP addresses to determine the best path for data?' would have the Network layer as the correct answer.
Why candidates choose this
Candidates may confuse the Network layer's role in packet fragmentation with the Transport layer's segmentation and sequencing, or mistakenly think that 'sequencing' relates to packet order in routing.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
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Last reviewed: Jun 30, 2026
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.
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