CCNA Network Infrastructure and Connectivity Practice Question
Which TWO statements correctly describe the encapsulation process at the OSI model Transport layer?
⚠ Common exam trap
Cisco often tests the distinction between OSI model layers, and the trap here is confusing the Transport layer's port numbers and segmentation with the Network layer's IP addressing or the Data Link layer's MAC addressing.
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
✓
It breaks data into smaller units called segments (for TCP) or datagrams (for UDP).
At the Transport layer (Layer 4), TCP segments data into segments and UDP segments data into datagrams. This segmentation allows efficient transmission and reassembly at the destination. The Transport layer header includes source and destination port numbers to identify the communicating applications, not IP or MAC 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.
- ✗
It adds source and destination IP addresses to the data.
Why it's wrong here
The Transport layer (Layer 4) does not add IP addresses; that is the function of the Network layer (Layer 3). IP addresses are logical addressing used for routable end-to-end delivery across internetworks, encapsulated in the IP header. The Transport layer instead focuses on managing communication between applications on hosts, using port numbers in its header to identify the services. Therefore, this option incorrectly assigns a Layer 3 responsibility to Layer 4.
- ✓
It breaks data into smaller units called segments (for TCP) or datagrams (for UDP).
Why this is correct
The Transport layer accepts data from upper layers and divides it into appropriately sized units for transmission. For TCP, these units are called segments, and for UDP, they are datagrams. This segmentation enables efficient network usage, and TCP further assigns sequence numbers to these units so the receiving host can reorder them correctly and request retransmission of missing data. This is a fundamental Transport layer role, ensuring that upper-layer data is properly prepared for network delivery.
- ✗
It adds a header that includes source and destination MAC addresses.
Why it's wrong here
MAC addresses are Layer 2 addresses, and they are added by the Data Link layer when it wraps a packet into a frame. The Transport layer header instead contains fields such as source and destination port numbers, sequence numbers (for TCP), and checksums. MAC addresses are only relevant for local delivery on a single physical network segment, whereas Transport layer information must survive across multiple hops from source to destination. Thus, this option describes a Layer 2 function, not a Layer 4 function.
- ✓
It adds a header that includes source and destination port numbers.
Why this is correct
A defining characteristic of the Transport layer header is the inclusion of source and destination port numbers, which are 16-bit values that uniquely identify the sending and receiving applications on the hosts. Port numbers allow the host's TCP/IP stack to demultiplex incoming traffic and deliver each stream to the correct process, such as a web server listening on port 80 or a DNS client using a random ephemeral port. This end-to-end application identification is not present in the network layer header, which only contains logical addresses. So this is a correct and essential function of the Transport layer.
- ✗
It converts data into bits for transmission over the physical medium.
Why it's wrong here
The conversion of data into bits and the actual transmission over the physical medium occurs at the Physical layer (Layer 1), which encodes frames into electrical, optical, or radio signals. The Transport layer works with abstract data units like segments and datagrams and never directly interfaces with hardware media. The Physical layer handles signaling, bit timing, and synchronization, while the Transport layer manages logical end-to-end communication. Therefore, this option misattributes a Layer 1 responsibility to 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.
✓It breaks data into smaller units called segments (for TCP) or datagrams (for UDP).Correct answer▾
Why this is correct
The Transport layer accepts data from upper layers and divides it into appropriately sized units for transmission. For TCP, these units are called segments, and for UDP, they are datagrams. This segmentation enables efficient network usage, and TCP further assigns sequence numbers to these units so the receiving host can reorder them correctly and request retransmission of missing data. This is a fundamental Transport layer role, ensuring that upper-layer data is properly prepared for network delivery.
✗It adds source and destination IP addresses to the data.Wrong answer — click to see why▾
Why this is wrong here
The Transport layer uses port numbers, not IP addresses. IP addresses are added during Network layer encapsulation.
✗It adds a header that includes source and destination MAC addresses.Wrong answer — click to see why▾
Why this is wrong here
MAC addresses are used for local delivery on a network segment and are part of Layer 2 encapsulation.
✗It converts data into bits for transmission over the physical medium.Wrong answer — click to see why▾
Why this is wrong here
The Physical layer handles the actual transmission of raw bits over the network medium.
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
TCP
TCP is a connection-oriented transport layer protocol that ensures reliable, ordered, and error-checked delivery of data between applications over IP networks.
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.
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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.