- A
PDU is a frame; IP address is added at the Data Link layer.
Why wrong: A frame is the PDU at the Data Link layer (Layer 2), not the Transport layer. IP addresses are added at the Network layer, not Data Link.
- B
PDU is a segment; IP address is added at the Network layer.
TCP segments are formed at the Transport layer, and the IP address is encapsulated at the Network layer when creating the packet.
- C
PDU is a packet; IP address is added at the Transport layer.
Why wrong: A packet is the PDU at the Network layer, not Transport. The IP address is added at the Network layer, not Transport.
- D
PDU is a datagram; IP address is added at the Transport layer.
Why wrong: A datagram is the PDU for UDP at the Transport layer, but the scenario does not specify TCP or UDP; however, the IP address is still added at the Network layer, not Transport.
OSI Model PDU Encapsulation — Segment, Packet, Frame
This 200-301 practice question tests your understanding of network infrastructure and connectivity. 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 connectivity issue between two hosts on different subnets. The sending host has constructed a packet with a destination IP address of 192.168.2.10. As the packet travels down the OSI model layers on the sending host, which Protocol Data Unit (PDU) name is assigned to the data at the Transport layer after TCP segments are created, and at which layer does the IP address get encapsulated?
Quick Answer
The answer is that the PDU at the Transport layer is a segment, and the destination IP address is encapsulated at the Network layer. This is correct because TCP, operating at the Transport layer, divides application data into segments by adding a TCP header, making the PDU a segment. The IP address is then added at the Network layer, where the IP header encapsulates that segment into a packet. On the CCNA 200-301 v2 exam, this concept tests your understanding of the OSI model’s encapsulation process and the specific PDU names at each layer. A common trap is confusing the Transport layer PDU for TCP (segment) with UDP (datagram) or mixing up where IP addresses are added—they belong at Layer 3, not Layer 4. A helpful memory tip is to think of the layers in order: data becomes a segment at Transport, then a packet at Network, and finally a frame at Data Link.
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
PDU is a segment; IP address is added at the Network layer.
At the Transport layer, TCP divides data into segments, so the PDU is called a segment, making B correct. The destination IP address (192.168.2.10) is added at the Network layer, where the IP header encapsulates the segment into a packet. Option A is wrong because a frame is a Data Link layer PDU, and IP addresses are not added at that layer. Option C is wrong because the PDU at the Transport layer is a segment, not a packet, and IP addresses are added at the Network layer, not the Transport layer. Option D is wrong because 'datagram' typically refers to UDP’s Transport layer PDU (not TCP), and IP addresses are not added at the Transport layer.
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.
- ✗
PDU is a frame; IP address is added at the Data Link layer.
Why it's wrong here
A frame is the PDU at the Data Link layer (Layer 2), not the Transport layer. IP addresses are added at the Network layer, not Data Link.
- ✓
PDU is a segment; IP address is added at the Network layer.
Why this is correct
TCP segments are formed at the Transport layer, and the IP address is encapsulated at the Network layer when creating the packet.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
PDU is a packet; IP address is added at the Transport layer.
Why it's wrong here
A packet is the PDU at the Network layer, not Transport. The IP address is added at the Network layer, not Transport.
- ✗
PDU is a datagram; IP address is added at the Transport layer.
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.
✓PDU is a segment; IP address is added at the Network layer.Correct answer▾
Why this is correct
TCP segments are formed at the Transport layer, and the IP address is encapsulated at the Network layer when creating the packet.
✗PDU is a frame; IP address is added at the Data Link layer.Wrong answer — click to see why▾
Why this is wrong here
The PDU at the Transport layer is a segment, not a frame. Frames are the PDU at the Data Link layer (Layer 2). Additionally, IP addresses are added at the Network layer (Layer 3), not the Data Link layer.
Why candidates choose this
Students often confuse the Data Link layer with the Network layer because both involve addressing (MAC vs. IP). The term 'frame' is commonly associated with encapsulation, leading to the mistaken belief that IP addresses are added at Layer 2.
✗PDU is a packet; IP address is added at the Transport layer.Wrong answer — click to see why▾
Why this is wrong here
A packet is the PDU at the Network layer (Layer 3), not the Transport layer. The IP address is added at the Network layer, not the Transport layer. The Transport layer PDU is a segment (for TCP) or a datagram (for UDP).
Why candidates choose this
The term 'packet' is widely used in networking and often misapplied to any encapsulated data unit. Students may incorrectly associate the IP address with the Transport layer because both are involved in end-to-end communication.
✗PDU is a datagram; IP address is added at the Transport layer.Wrong answer — click to see why▾
Why this is wrong here
A datagram is the PDU for UDP at the Transport layer, but the question specifies TCP segments. Even if it were UDP, the IP address is still added at the Network layer, not the Transport layer.
Why candidates choose this
Students may recall that UDP uses datagrams and think that the IP address is part of the Transport layer encapsulation. However, IP addressing is always a Network layer function, regardless of the Transport protocol.
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?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the precise PDU naming per layer (segment for TCP at Transport, packet for IP at Network) and the layer where IP addresses are added, tricking candidates who confuse 'packet' with 'segment' or think IP addresses are added at the Transport layer.
Trap categories for this question
Scenario analysis trap
A datagram is the PDU for UDP at the Transport layer, but the scenario does not specify TCP or UDP; however, the IP address is still added at the Network layer, not Transport.
Detailed technical explanation
How to think about this question
In the OSI model, each layer adds its own header: the Transport layer (Layer 4) encapsulates data into segments (TCP) or datagrams (UDP), while the Network layer (Layer 3) adds the source and destination IP addresses to form a packet. This encapsulation process ensures that the IP address is available for routing decisions at Layer 3, and the segment is carried as the payload of the packet. A real-world scenario is when a host sends a TCP SYN to 192.168.2.10: the segment is built first, then the IP header with the destination address is added before the frame is created.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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 |
What to study next
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FAQ
Questions learners often ask
What does this 200-301 question test?
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: PDU is a segment; IP address is added at the Network layer. — At the Transport layer, TCP divides data into segments, so the PDU is called a segment, making B correct. The destination IP address (192.168.2.10) is added at the Network layer, where the IP header encapsulates the segment into a packet. Option A is wrong because a frame is a Data Link layer PDU, and IP addresses are not added at that layer. Option C is wrong because the PDU at the Transport layer is a segment, not a packet, and IP addresses are added at the Network layer, not the Transport layer. Option D is wrong because 'datagram' typically refers to UDP’s Transport layer PDU (not TCP), and IP addresses are not added at the Transport layer.
What should I do if I get this 200-301 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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more ways this is tested on 200-301
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
medium- A.Network layer
- ✓ B.Transport layer
- C.Data Link layer
- D.Application layer
Why B: 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.
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Last reviewed: Jun 11, 2026
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