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CCNA Network Infrastructure and Connectivity Practice Question
Drag and drop the following steps into the correct order to describe the data encapsulation process as it flows down the OSI model from the source host.
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
✓
Application data is generated and passed to the Transport layer.
Encapsulation proceeds from the top OSI layers to the bottom. First, the application generates data (Step 1). The Transport layer then adds a header (TCP or UDP) to create a segment (Step 2) because this is the first step in preparing data for reliable delivery. The Network layer adds an IP header to form a packet (Step 3), which provides logical addressing and routing information. The Data Link layer adds a frame header and trailer to create a frame (Step 4), enabling physical addressing (MAC) and error detection. Finally, the Physical layer converts the frame to bits for transmission (Step 5), as it is the layer that ultimately puts signals on the wire.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Application data is generated and passed to the Transport layer.
Why this is correct
At the source host, the application layer process (e.g., a web browser or email client) creates the actual user payload, such as an HTTP GET request or SMTP message. This raw data is then handed downward to the Transport layer, where it becomes the payload of a future segment. The application layer itself does not add any headers that are relevant to the network delivery process; it only formats the data for the application protocol.
- ✓
The Transport layer encapsulates the data by adding a TCP or UDP header, creating a segment.
Why this is correct
The Transport layer (typically TCP or UDP) takes the application payload and prepends a header containing source and destination port numbers, along with sequencing, acknowledgment, or checksum fields (in the case of TCP). This newly created protocol data unit is called a segment. For TCP, the segment header also includes window size and flags to manage reliable delivery, whereas UDP's 8-byte header only carries ports, length, and checksum, reflecting its connectionless nature.
- ✓
The Network layer adds an IP header to the segment, forming a packet.
Why this is correct
At the Network layer, the IP protocol encapsulates the incoming segment by appending its own 20- to 60-byte header, which includes logical source and destination IP addresses, a TTL field, protocol number (6 for TCP, 17 for UDP), and a header checksum (IPv4). This layer's protocol data unit is called a packet, and it is the IP addresses in this header that allow the packet to be routed across different networks, not the MAC addresses used later.
- ✓
The Data Link layer adds a frame header and trailer, creating a frame.
Why this is correct
The Data Link layer, typically Ethernet, encapsulates the IP packet by adding a frame header (preamble/delimiter, destination and source MAC addresses, EtherType) and a trailer (Frame Check Sequence or FCS). This creates a frame, which is the protocol data unit that can be physically transmitted on a local network segment. The FCS allows the receiving device to detect any bit errors introduced during transmission, while the MAC addresses enable delivery to a specific NIC on the same LAN.
- ✓
The Physical layer converts the frame into bits and transmits them onto the medium.
Why this is correct
The Physical layer takes the digital frame bits and converts them into signals appropriate for the transmission medium, such as electrical voltages on copper cable, light pulses on fiber, or radio waves in wireless. This bitstream is encoded according to the physical layer standard (e.g., 1000BASE-T or 802.11ac) and transmitted onto the medium. At this point, no further headers or trailers are added—only line coding and modulation are performed to ensure the bits can travel across the physical link.
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
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
Internet Protocol
Internet Protocol (IP) is the set of rules that governs how data is addressed, routed, and sent from one device to another across networks, including the internet.
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Written by Johnson Ajibi, MSc IT Security
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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.