CCNA Network Infrastructure and Connectivity Practice Question

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 user application.

  Why this is correct

  This statement is correct and describes the starting point of the encapsulation process. The user application, such as a browser or email client, generates the original payload—for example, HTTP request text or SMTP message content—which exists as an application-layer PDU called "data." At this stage no transport, network, or data-link header has been added, so this raw information must come first before any lower-layer encapsulation can occur.

• ✓

  The transport layer encapsulates the data into a segment by adding a TCP or UDP header.

  Why this is correct

  This statement is correct: the transport layer takes the application data and adds a TCP or UDP header, creating a segment (or UDP datagram). The header carries essential end-to-end information such as source and destination port numbers, and for TCP it also adds sequence numbers, acknowledgments, and checksums for reliable delivery. Without this layer, the receiving host could not demultiplex the data to the correct application.

• ✓

  The network layer encapsulates the segment into a packet by adding an IP header.

  Why this is correct

  This statement is correct: the network layer encapsulates the segment by prepending an IP header that contains logical source and destination IP addresses, a TTL field, and a protocol identifier indicating whether the payload is TCP or UDP. The resulting PDU is called a packet (or IP datagram). This IP header enables routers to make forwarding decisions and deliver the packet across multiple layer-3 networks.

• ✓

  The data link layer encapsulates the packet into a frame by adding a frame header and trailer.

  Why this is correct

  This statement is correct: the data link layer wraps the packet in a frame by adding an L2 header—such as Ethernet's source and destination MAC addresses and type field—and a trailer, typically a Frame Check Sequence (FCS) used for error detection. This frame format allows the packet to travel over a specific physical link, and the trailer enables receiving interfaces to detect corrupted transmissions.

• ✓

  The physical layer converts the frame into bits and transmits them over the medium.

  Why this is correct

  This statement is correct and represents the final outbound encapsulation step: the physical layer encodes each bit of the frame using electrical, optical, or radio signaling and transmits those bits over the medium. It does not add headers or trailers; it simply converts the complete frame into a bit stream on the physical wire, fiber, or wireless channel. This is where the frame leaves the sending device and enters the network.

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