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CCNA Network Infrastructure and Connectivity Practice Question
Drag and drop the following steps into the correct order to describe how data is encapsulated as it travels down the OSI model layers.
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 by the upper layers.
Data originates at the application layer as user data. The transport layer then encapsulates this data by adding a header (e.g., TCP or UDP) to create a segment. The network layer further adds an IP header to the segment, forming a packet. Next, the data link layer appends a frame header and trailer to the packet, resulting in a frame. Finally, the physical layer converts the entire frame into bits for electrical, optical, or radio transmission. This sequence follows the standard top-down encapsulation process in network communication.
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 by the upper layers.
Why this is correct
At the top of the protocol stack, the application layer (Layer 7) produces the actual user payload—such as an HTTP request or DNS query—before any header is applied. This original data stream then moves downward through the OSI model, where each successive layer appends its own control information. In other words, upper-layer application data is the foundational content that ultimately becomes a segment, packet, frame, and then bits on the wire.
- ✓
The transport layer adds a header to create a segment.
Why this is correct
During encapsulation, the transport layer in both the OSI and TCP/IP models takes the application-layer data and prepends either a TCP or UDP header, forming a segment (or datagram). This header includes critical fields such as source and destination port numbers, sequence numbers (for TCP), and checksum data to ensure end-to-end delivery and, when TCP is used, reliability. The result is called a segment specifically because the payload is now wrapped with L4 transport control information.
- ✓
The network layer adds an IP header to create a packet.
Why this is correct
The network layer, or Internet layer, receives the segment and wraps it with an IP header, turning it into a packet. This header holds logical addressing (source/destination IPv4 or IPv6 addresses), a protocol identifier, and a TTL/hop-limit field that supports global routing. Routers inspect these IP addresses to decide the next hop, so the packet is the decisive PDU for path selection across internetworks.
- ✓
The data link layer adds a header and trailer to create a frame.
Why this is correct
At Layer 2, the data link layer places the IP packet inside a frame by adding a header with MAC addresses and a trailer containing the Frame Check Sequence, which is used for error detection. Ethernet is the most common framing standard, and the trailer is just as important as the header because it verifies that the frame arrived intact after physical transmission. Thus the frame is the PDU that traverses a single subnet or LAN segment.
- ✓
The physical layer converts the frame into bits for transmission.
Why this is correct
The physical layer performs the final conversion, taking the completed frame and encoding all of its bits as electrical voltages, light pulses, or radio waves, depending on the medium such as copper, fiber, or wireless. No new header or trailer is added here—this layer is only concerned with bit transmission, bit timing, and physical signaling rates. Once the bits are transmitted across the medium, the receiving device reverses the process, decoding bits, then frames, packets, and segments, until the original application data is reassembled.
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
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.
Key term
OSI model
The OSI model is a conceptual framework that standardizes the functions of a telecommunication or computing system into seven distinct layers, from physical hardware to application software.
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