CCNA Network Infrastructure and Connectivity Practice Question
Which TWO statements accurately describe the encapsulation process in the TCP/IP model as data moves from the application layer to the network access layer?
⚠ Common exam trap
Cisco often tests the specific PDU naming conventions (segment, packet, frame) and the layer at which each header is added, causing candidates to confuse the network layer packet with the data link layer frame or to misidentify the transport layer PDU.
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
✓
At the transport layer, the PDU is called a segment (for TCP) and includes source and destination port numbers.
At the transport layer, TCP creates segments that include source and destination port numbers (B correct). At the network layer, the PDU is a packet containing source and destination IP addresses (D correct). Option A is wrong because the application layer generates data, not segments, and transport headers are added later. Option C mislabels the network layer PDU; it is a packet, not a frame, and MAC addresses belong to frames. Option E is wrong because the data link layer PDU is a frame, not a packet, and it uses MAC addresses, not IP addresses.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
At the application layer, the PDU is called a segment and includes a transport layer header.
Why it's wrong here
The application layer PDU is simply called data, and it has no transport layer header; that header is added later by the transport layer when it encapsulates the application data into a segment. A segment, with source and destination port numbers, is the transport layer PDU for TCP. Therefore, calling the application layer unit a segment and assigning it a transport header conflates two distinct layers of the encapsulation process.
- ✓
At the transport layer, the PDU is called a segment (for TCP) and includes source and destination port numbers.
Why this is correct
The transport layer PDU for TCP is a segment, and its header carries source and destination port numbers to identify the sending and receiving applications, along with sequence and acknowledgment numbers for reliability. This segmentation prepares data for network-layer encapsulation. For UDP, the equivalent PDU is called a datagram, so the term segment specifically applies to TCP.
- ✗
At the network layer, the PDU is called a frame and includes source and destination MAC addresses.
Why it's wrong here
The network layer PDU is a packet, not a frame, and it contains IP addresses for routing, not source and destination MAC addresses. MAC addresses are used in data link layer frames for delivery within a single network segment or VLAN. This option incorrectly swaps the PDU name and addressing of the network and data link layers.
- ✓
At the network layer, the PDU is called a packet and includes source and destination IP addresses.
Why this is correct
At the network layer, the PDU is indeed a packet, which is the encapsulated transport layer segment preceded by an IP header containing the source and destination IP addresses. This logical addressing allows routers to forward the packet end-to-end across multiple networks. The packet is the fundamental unit of network-layer forwarding and is created by the IP protocol.
- ✗
At the data link layer, the PDU is called a packet and includes source and destination IP addresses.
Why it's wrong here
The data link layer PDU is a frame, not a packet, and it uses hardware addresses such as MAC addresses, not logical IP addresses. A packet belongs to the network layer and carries IP routing information. Frames handle local segment delivery, so equating the data link PDU with a packet and IP addresses misidentifies both the protocol data unit and its addressing scheme.
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.
✓At the transport layer, the PDU is called a segment (for TCP) and includes source and destination port numbers.Correct answer▾
Why this is correct
The transport layer PDU for TCP is a segment, and its header carries source and destination port numbers to identify the sending and receiving applications, along with sequence and acknowledgment numbers for reliability. This segmentation prepares data for network-layer encapsulation. For UDP, the equivalent PDU is called a datagram, so the term segment specifically applies to TCP.
✗At the application layer, the PDU is called a segment and includes a transport layer header.Wrong answer — click to see why▾
Why this is wrong here
The application layer PDU is called data, not a segment. A segment is the transport layer PDU, and it includes a transport layer header, not an application layer header.
Why candidates choose this
Students often confuse the terms 'segment' and 'data' because they know that segmentation occurs at the transport layer, but they may incorrectly associate the term with the application layer.
✗At the network layer, the PDU is called a frame and includes source and destination MAC addresses.Wrong answer — click to see why▾
Why this is wrong here
The network layer PDU is called a packet, not a frame. Frames are at the data link layer and include MAC addresses, not IP addresses.
Why candidates choose this
Students may confuse the terms 'packet' and 'frame' because both are used in networking, and they might think that frames contain IP addresses since they are used in local delivery.
✗At the data link layer, the PDU is called a packet and includes source and destination IP addresses.Wrong answer — click to see why▾
Why this is wrong here
The data link layer PDU is called a frame, not a packet. Frames include source and destination MAC addresses, not IP addresses.
Why candidates choose this
Students often mix up the terms 'packet' and 'frame' because both are encapsulated units, and they may incorrectly associate IP addresses with the data link layer due to their role in end-to-end communication.
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
Packet
A packet is a small unit of data that is sent over a network, containing both the actual data and control information for delivery.
Key term
Encapsulation
Encapsulation is the process of wrapping data with protocol headers and trailers before sending it across a network.
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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.