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.

• ✗

  During encapsulation, the transport layer adds a header to form a packet.

  Why it's wrong here

  Although the transport layer does add a header during encapsulation, the resulting protocol data unit is not called a packet. A packet is the PDU at the network layer; the transport layer produces a segment (for TCP) or a datagram (for UDP). The transport header contains port information to support process-to-process communication, but calling it a packet misidentifies the layer where the encapsulation occurs.

• ✓

  During encapsulation, the transport layer adds a header containing source and destination port numbers.

  Why this is correct

  During encapsulation, the transport layer encapsulates upper-layer data with a header that records source and destination port numbers. These ports identify the specific application processes on the sending and receiving hosts, enabling the OS to demultiplex incoming data to the correct service. In TCP, the header includes additional fields like sequence numbers and windows, but the port numbers are the essential addressing mechanism that distinguishes this layer from the network layer's IP addressing.

• ✗

  During de-encapsulation, the transport layer removes the IP header to extract the segment.

  Why it's wrong here

  De-encapsulation at the receiving host is a layered process, and removing the IP header is the network layer's job, not the transport layer's. Once the network layer strips the IP header from the packet, it hands the remaining segment to the transport layer. The transport layer then examines its own header—for example, verifying the TCP checksum and reading destination ports—before processing the payload. Therefore, this statement incorrectly attributes the network layer's function to the transport layer.

• ✓

  During de-encapsulation, the transport layer removes its header and passes the payload to the session layer.

  Why this is correct

  In the OSI model, after the transport layer has verified its header and performed any reliability functions, it removes that header and passes the payload upward to the session layer (Layer 5). The transport layer is the first layer that provides end-to-end communication between applications, so it hands data to the session layer after ensuring it has been properly received. Note that in the TCP/IP model the analogous handoff is to the application layer, but the question's context follows OSI terminology, making this statement correct.

• ✗

  During encapsulation, the transport layer adds a trailer for error detection.

  Why it's wrong here

  Error-detection trailers are not a transport-layer function; the data link layer appends a frame check sequence, such as a CRC trailer, at the end of a frame. The transport layer, particularly TCP, does include a checksum field inside its header to detect bit errors in the segment, but this is not a trailer. Adding a trailer at Layer 4 would be redundant and would conflict with how PDU boundaries are maintained in layer 2 framing.

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.

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?”

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