CCNA Transport Layer (Layer 4) Practice Question
Which single OSI model layer is responsible for both end‑to‑end reliable data delivery and segmenting data into smaller units?
⚠ Common exam trap
Candidates may think the Network layer also performs both functions because it segments data into packets and provides end‑to‑end delivery, but reliability is strictly a Transport layer function. The question asks for a single layer, so combining Transport and Network is incorrect.
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 (Layer 4)
The Transport layer (Layer 4) is the only OSI layer responsible for both end‑to‑end reliable data delivery (via protocols like TCP) and segmenting data into smaller units (segments). The Network layer (Layer 3) handles routing and packetizing but does not provide reliability. Therefore, only the Transport layer fulfills both requirements.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Transport layer (Layer 4)
Why this is correct
The Transport layer (Layer 4) is correct: it accepts data from the Session layer, divides it into segments, and is responsible for end-to-end delivery. TCP, a Transport layer protocol, provides reliable delivery via sequence numbers, acknowledgments, timeouts, and retransmissions, while UDP is a simpler, unreliable transport alternative. This layer is uniquely positioned to ensure that the entire message arrives intact and in order across the network.
- ✗
Data Link layer (Layer 2)
Why it's wrong here
The Data Link layer (Layer 2) is wrong because it only delivers data between two directly connected nodes on the same local network, using MAC addresses and frames. It does not provide end-to-end delivery across multiple networks, which is a Network layer function, nor does it offer reliable transport service such as acknowledgment and retransmission. L2 error detection (e.g., FCS) can discard corrupt frames, but it does not recover lost data, so it cannot ensure reliable delivery.
- ✗
Network layer (Layer 3)
Why it's wrong here
The Network layer (Layer 3) is wrong because it provides logical addressing (IP) and routes packets end-to-end, but it does not guarantee reliable delivery. IP is a best-effort, connectionless protocol: it fragments datagrams when they exceed the MTU (fragmentation), not segments application data into segments, and leaves reliability to the upper layers. Thus, the Network layer neither performs the segmentation described in the question nor offers the reliability guarantee.
- ✗
Session layer (Layer 5)
Why it's wrong here
The Session layer (Layer 5) is wrong because it manages and controls the dialog between two hosts by establishing, maintaining, and terminating sessions, not by moving or segmenting data. It can provide synchronization points and checkpointing, but it does not guarantee delivery or break data into transport units. Session-layer functions operate above the delivery model, so reliability and segmentation are outside its scope.
- ✗
Transport layer (Layer 4) and Network layer (Layer 3)
Why it's wrong here
Incorrect. The question asks for a single layer. Although both layers contribute, only the Transport layer provides both reliable delivery and segmentation. The Network layer lacks reliability, and listing two layers is not a single layer.
When this WOULD be correct
In real enterprise networks, the Transport layer (Layer 4) ensures end-to-end reliable delivery (e.g., TCP) and segments data into segments. The Network layer (Layer 3) provides logical addressing and routing of packets, which is essential for end-to-end delivery across multiple networks. Together, they directly support end-to-end communication, though segmentation is exclusively a Transport layer function.
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 layer (Layer 4)Correct answer▾
Why this is correct
The Transport layer (Layer 4) is correct: it accepts data from the Session layer, divides it into segments, and is responsible for end-to-end delivery. TCP, a Transport layer protocol, provides reliable delivery via sequence numbers, acknowledgments, timeouts, and retransmissions, while UDP is a simpler, unreliable transport alternative. This layer is uniquely positioned to ensure that the entire message arrives intact and in order across the network.
✗Data Link layer (Layer 2)Wrong answer — click to see why▾
Why this is wrong here
The Data Link layer (Layer 2) handles node-to-node delivery within the same network segment, not end-to-end across multiple networks. It frames data into frames and adds physical addressing (MAC addresses), but does not segment data from upper layers or provide reliable end-to-end delivery.
Why candidates choose this
Students might confuse the Data Link layer's framing with segmentation, as both involve breaking data into smaller units. However, framing occurs at Layer 2 and is for local delivery, while segmentation at Layer 4 is for end-to-end transport.
✗Network layer (Layer 3)Wrong answer — click to see why▾
Why this is wrong here
The Network layer (Layer 3) is responsible for logical addressing (IP addresses) and routing packets across networks, but it does not provide reliable end-to-end delivery or segment data. Reliable delivery and segmentation are functions of the Transport layer.
Why candidates choose this
Students might think the Network layer handles segmentation because it deals with packets, but packets are the PDU of Layer 3 and are created from segments. The actual segmentation occurs at Layer 4.
✗Session layer (Layer 5)Wrong answer — click to see why▾
Why this is wrong here
The Session layer (Layer 5) manages sessions, including establishing, maintaining, and terminating connections between applications. It does not segment data or ensure reliable delivery; those are Transport layer functions.
Why candidates choose this
Students might associate 'session' with end-to-end communication and mistakenly think the Session layer handles reliable delivery. However, reliability is a Transport layer responsibility.
✗Transport layer (Layer 4) and Network layer (Layer 3)Wrong answer — click to see why▾
Why this is wrong here
Incorrect. The question asks for a single layer. Although both layers contribute, only the Transport layer provides both reliable delivery and segmentation. The Network layer lacks reliability, and listing two layers is not a single layer.
★ When this WOULD be the correct answer
In real enterprise networks, the Transport layer (Layer 4) ensures end-to-end reliable delivery (e.g., TCP) and segments data into segments. The Network layer (Layer 3) provides logical addressing and routing of packets, which is essential for end-to-end delivery across multiple networks. Together, they directly support end-to-end communication, though segmentation is exclusively a Transport layer function.
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?”
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
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.
Key term
Internet Protocol
Internet Protocol (IP) is the set of rules that governs how data is addressed, routed, and sent from one device to another across networks, including the internet.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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