CCNA Network Infrastructure and Connectivity Practice Question
Which OSI layer is responsible for end-to-end segmentation, port numbers, and reliability functions such as acknowledgments?
⚠ Common exam trap
Don't confuse the Transport Layer's end-to-end functions with the Network Layer's routing or the Data Link Layer's local communication roles.
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
The Transport layer (Layer 4) is responsible for end-to-end segmentation, port numbers for identifying applications, and reliability functions like acknowledgments and retransmission. The Network layer (Layer 3) handles logical addressing and routing between networks. The Data Link layer (Layer 2) manages local frame delivery and error detection. The Session layer (Layer 5) controls dialog management and synchronization, not segmentation or ports.
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
Why it's wrong here
The network layer is responsible for logical addressing (IP addresses) and routing decisions, determining the best path across multiple networks. It encapsulates segments into packets and forwards them based on destination IP, but it does not perform the end-to-end segmentation of user data nor does it use port numbers. Ports exist only at the transport layer, so while the network layer works end-to-end in a routed sense, it does not provide process-to-process delivery.
When this WOULD be correct
If the question asked which OSI layer is responsible for logical addressing and routing of packets across networks, the correct answer would be Network. This would focus on the functions of the Network layer in facilitating communication between different devices on separate networks.
- ✗
Data Link
Why it's wrong here
The data link layer operates on a single physical segment or link, using framing and MAC addresses to move frames between directly connected devices. It provides error detection via checksums (e.g., FCS) but lacks any concept of ports for process identification. Because it only handles hop-to-hop delivery, it cannot perform the end-to-end segmentation that occurs at the transport layer.
When this WOULD be correct
If the question asked about the layer responsible for framing, MAC addressing, and physical addressing in a local network context, then the Data Link layer would be the correct answer, as it manages these functions between directly connected devices.
- ✓
Transport
Why this is correct
At the transport layer, protocols such as TCP and UDP perform end-to-end segmentation, breaking application data into segments that can be reassembled at the destination. The transport header carries source and destination port numbers, which identify the specific application processes at each end of the communication session. TCP also adds reliability mechanisms like sequencing and acknowledgments, making it the definitive layer for this function.
- ✗
Session
Why it's wrong here
The session layer is responsible for establishing, managing, and terminating dialog between network applications, including checkpoints and recovery. It does not define TCP reliability features such as sequencing or retransmission; those are transport-layer mechanisms. Ports and segment fragmentation are likewise outside the session layer's scope, making it an incorrect answer for this function.
When this WOULD be correct
If the question asked which OSI layer is responsible for managing sessions and maintaining communication between applications, then the correct answer would be the Session layer, as it focuses on the control and management of ongoing sessions.
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.
✓TransportCorrect answer▾
Why this is correct
At the transport layer, protocols such as TCP and UDP perform end-to-end segmentation, breaking application data into segments that can be reassembled at the destination. The transport header carries source and destination port numbers, which identify the specific application processes at each end of the communication session. TCP also adds reliability mechanisms like sequencing and acknowledgments, making it the definitive layer for this function.
✗NetworkWrong 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 between networks, not for end-to-end segmentation, port numbers, or reliability functions like acknowledgments.
★ When this WOULD be the correct answer
If the question asked which OSI layer is responsible for logical addressing and routing of packets across networks, the correct answer would be Network. This would focus on the functions of the Network layer in facilitating communication between different devices on separate networks.
Why candidates choose this
Students often confuse the network layer with the transport layer because both deal with end-to-end delivery, but the network layer focuses on path determination and addressing, not on reliability or port numbers.
✗Data LinkWrong answer — click to see why▾
Why this is wrong here
The data link layer (Layer 2) handles framing, MAC addressing, and error detection on a single local network segment, not end-to-end segmentation or port numbers.
★ When this WOULD be the correct answer
If the question asked about the layer responsible for framing, MAC addressing, and physical addressing in a local network context, then the Data Link layer would be the correct answer, as it manages these functions between directly connected devices.
Why candidates choose this
Since the data link layer provides error detection (e.g., CRC), some students might mistakenly think it also handles reliability functions like acknowledgments, but those are transport layer functions.
✗SessionWrong 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, but it does not handle segmentation, port numbers, or reliability functions like acknowledgments.
★ When this WOULD be the correct answer
If the question asked which OSI layer is responsible for managing sessions and maintaining communication between applications, then the correct answer would be the Session layer, as it focuses on the control and management of ongoing sessions.
Why candidates choose this
Because the session layer deals with managing communication sessions, some students might incorrectly associate it with reliability functions, but those are handled by 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
IPv4 Addressing and Address Classes
Key term
Frame
A frame is a unit of data at the Data Link layer of networking, containing header, payload, and trailer information for local network delivery.
Key term
Functions
A function is a reusable block of code that performs a specific task, taking inputs, processing them, and returning a result, helping to organize and automate IT workflows.
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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.