CCNA Network Infrastructure and Connectivity Practice Question
Which TWO statements correctly describe the OSI model layers and their corresponding PDU names during encapsulation?
⚠ Common exam trap
Cisco often tests the specific PDU names at each layer (e.g., segment vs. packet vs. frame vs. bit) and the order of encapsulation, so the trap here is confusing the Data Link layer PDU (frame) with bits or mixing up the Session layer's role with encapsulation.
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
✓
The Transport layer encapsulates data into segments.
Both A and B are correct. During encapsulation, the Transport layer (Layer 4) adds a header to upper-layer data to form segments, and the Network layer (Layer 3) encapsulates those segments into packets by adding a network-layer header (e.g., IP). Option C is incorrect because the Data Link layer encapsulates packets into frames, not bits; bits are the raw transmission unit at the Physical layer. Option D is incorrect because the Session layer manages sessions but does not produce a PDU called 'sessions'; encapsulation of segments only occurs at Layers 4 and below. Option E is incorrect because the Physical layer transmits frames as bits but does not perform encapsulation; encapsulation ends at the Data Link layer.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The Transport layer encapsulates data into segments.
Why this is correct
The Transport layer encapsulates upper-layer data (from the Session layer or Application layer) by adding a TCP or UDP header to create a segment. This header includes source and destination port numbers, and for TCP also sequence numbers, acknowledgment numbers, and window size, enabling reliable end-to-end delivery. This process of dividing data and prepending transport control information is exactly what produces the segment PDU, making this statement correct.
- ✓
The Network layer encapsulates segments into packets.
Why this is correct
The Network layer performs encapsulation by receiving segments from the Transport layer and adding an IP header to form a packet. This header contains logical addressing, such as source and destination IP addresses, along with protocol identification and TTL fields. The resulting Protocol Data Unit (PDU) is called a packet, and it is the unit that is routed across internetworks. This statement accurately describes Layer 3 encapsulation.
- ✗
The Data Link layer encapsulates packets into bits.
Why it's wrong here
The Data Link layer correctly performs encapsulation when it wraps Network-layer packets with a frame header and trailer, such as MAC addresses and a Frame Check Sequence. The resulting PDU is called a frame, not bits. The Physical layer is the one responsible for transmitting those frames as bits or signals over the wire. Thus, the error lies in the final PDU name—the Data Link layer encapsulates packets into frames, and the physical layer handles bit encoding.
- ✗
The Session layer encapsulates segments into sessions.
Why it's wrong here
The Session layer (Layer 5) is responsible for establishing, managing, and terminating communication sessions—dialog control—between applications. It does not encapsulate data into a 'session' PDU; neither the OSI nor TCP/IP model defines such a PDU at this layer. Instead, the session layer works with the data it receives and, in practice, the PDU at Layers 5 through 7 is commonly called 'data.' Encapsulation at this layer is not standard, and there is no 'session' header added, so the statement is incorrect.
- ✗
The Physical layer encapsulates frames into bits.
Why it's wrong here
The Physical layer does not encapsulate frames into bits; encapsulation implies the addition of headers and trailers as data moves down the protocol stack. The Physical layer instead receives frames from the Data Link layer and converts their contents into raw bits, signaling elements, or pulses suitable for the transmission medium. It is a transmission function, not an encapsulation function, and the PDU at this layer is simply bits or symbols. Therefore, saying it 'encapsulates' frames into bits misrepresents both the process and the terminology.
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.
✓The Transport layer encapsulates data into segments.Correct answer▾
Why this is correct
The Transport layer encapsulates upper-layer data (from the Session layer or Application layer) by adding a TCP or UDP header to create a segment. This header includes source and destination port numbers, and for TCP also sequence numbers, acknowledgment numbers, and window size, enabling reliable end-to-end delivery. This process of dividing data and prepending transport control information is exactly what produces the segment PDU, making this statement correct.
✗The Data Link layer encapsulates packets into bits.Wrong answer — click to see why▾
Why this is wrong here
The Data Link layer encapsulates packets into frames, not bits; bits are the PDU of the Physical layer.
✗The Session layer encapsulates segments into sessions.Wrong answer — click to see why▾
Why this is wrong here
The Session layer handles session management and does not perform encapsulation into 'sessions'; encapsulation at this layer is not defined.
✗The Physical layer encapsulates frames into bits.Wrong answer — click to see why▾
Why this is wrong here
The Physical layer transmits frames as bits but does not encapsulate frames into bits; encapsulation ends at Layer 2.
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
Encapsulation
Encapsulation is the process of wrapping data with protocol headers and trailers before sending it across a network.
About these practice questions
One of 1,389 original 200-301 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
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.