CCNA Network Infrastructure and Connectivity Practice Question
A network technician is troubleshooting a connectivity issue between two hosts on different subnets. During the analysis, the technician captures packets and observes that the data link layer frames are being stripped and rebuilt at each router hop. Which layer of the OSI model is responsible for encapsulating the original data into segments before transmission from the source host?
⚠ Common exam trap
Cisco often tests the distinction between encapsulation layers by describing a Layer 2 behavior (frame stripping/rebuilding) in the scenario to mislead candidates into selecting the Data Link layer, when the question specifically asks about the layer that creates segments at the source host.
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
✓
Transport layer
The Transport layer (Layer 4) is responsible for encapsulating the original data into segments. Protocols such as TCP (RFC 793) or UDP (RFC 768) add a header containing source and destination port numbers, sequence numbers, and other control information to form a segment. This segmentation occurs at the source host before the data is passed down to the Network layer for routing.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Network layer
Why it's wrong here
The Network layer operates with packets and logical IP addressing, performing route selection and forwarding between different networks. However, it does not segment upper-layer data; rather, it encapsulates the Transport layer's segments into packets. Because segmentation occurs at Layer 4, this layer's responsibility is path determination, not creating segments.
- ✓
Transport layer
Why this is correct
The Transport layer is the exact location where upper-layer application data is broken into smaller units and a transport header is appended, producing TCP segments or UDP datagrams. It manages end-to-end communications, including flow control and reliability, using port numbers and, in TCP, sequence and acknowledgment numbers. This segmentation function is the defining responsibility of Layer 4.
- ✗
Data Link layer
Why it's wrong here
The Data Link layer handles local network delivery by converting Network layer packets into frames and attaching MAC addresses and a frame check sequence for error detection. It does not segment application data; instead, it assumes the packet it receives already contains a Transport layer segment. Because segmentation occurs at Layer 4, the frame-building process at Layer 2 cannot be the correct answer.
- ✗
Application layer
Why it's wrong here
The Application layer serves as the interface between user applications and the network, generating data and providing protocols such as HTTP, SMTP, and FTP. It neither adds transport headers nor splits data into segments; instead, it hands off the complete data stream to the Transport layer for segmentation. Its focus is on data origin and process-to-process communication, not on constructing segments.
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 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Transport layerCorrect answer▾
Why this is correct
The Transport layer is the exact location where upper-layer application data is broken into smaller units and a transport header is appended, producing TCP segments or UDP datagrams. It manages end-to-end communications, including flow control and reliability, using port numbers and, in TCP, sequence and acknowledgment numbers. This segmentation function is the defining responsibility of Layer 4.
✗Network layerWrong answer — click to see why▾
Why this is wrong here
The Network layer (Layer 3) encapsulates segments into packets and adds logical addressing (IP addresses) for routing across networks, but it does not perform the initial segmentation of data into segments.
Why candidates choose this
Students often confuse the Network layer with the Transport layer because both deal with addressing and encapsulation, but the Network layer works with packets, not segments.
✗Data Link layerWrong answer — click to see why▾
Why this is wrong here
The Data Link layer (Layer 2) encapsulates packets into frames and adds physical addressing (MAC addresses) for delivery on a local network segment, but it does not perform segmentation of data into segments.
Why candidates choose this
Since the question mentions frames being stripped and rebuilt at each router hop, students might mistakenly think the Data Link layer is responsible for the initial segmentation, but that is the Transport layer's role.
✗Application layerWrong answer — click to see why▾
Why this is wrong here
The Application layer (Layer 7) provides the interface for applications to generate data, but it does not perform segmentation or encapsulation into segments. Segmentation occurs at the Transport layer.
Why candidates choose this
Students may think that because applications generate data, the Application layer is responsible for all encapsulation, but the OSI model assigns segmentation to the Transport layer.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
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 |
Go deeper
Related to this question
Learn chapter
The OSI Model
Key term
UDP
UDP (User Datagram Protocol) is a communication protocol that sends data quickly without first checking if the receiver is ready or if the data arrived correctly.
Key term
Open Systems Interconnection
The Open Systems Interconnection (OSI) model is a conceptual framework used to understand how data travels from one computer to another across a network.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco 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 200-301 exam.