Which OSI layer adds the destination port number to a PDU during encapsulation?

• A. Data Link Layer
• B. Transport Layer
• C. Network Layer
• D. Application Layer

Answer: (B)

Port addressing for delivering data to the correct application is handled at the Transport Layer. As data from higher layers is encapsulated, the Transport Layer creates a transport segment and adds a header containing the destination port number (along with the source port). That destination port tells the receiving host which process should receive the data, enabling multiplexing multiple communications over a single connection. The Data Link Layer uses MAC addresses for local delivery, the Network Layer adds IP addresses for routing, and the Application Layer doesn’t insert port numbers into the PDU. So, the layer that adds the destination port number during encapsulation is the Transport Layer.