At which OSI layer is a destination port number added to a PDU during the encapsulation process?

During the encapsulation process, a destination port number is added to a Protocol Data Unit (PDU) at the Transport layer of the OSI model. The Transport layer is responsible for providing end-to-end communication services for applications. By adding a destination port number, the Transport layer ensures that the data is delivered to the correct application on the target host. This layer manages the segmentation of data and facilitates reliable or unreliable communication depending on the protocol used (e.g., TCP for reliable transmission or UDP for faster, connectionless transmission).

The other layers do not involve the addition of port numbers. The Network layer focuses on logical addressing and routing, while the Data link layer deals with physical addressing and frame encapsulation for transmission between devices on the same local network. The Session layer manages sessions between applications but does not handle port numbers. Thus, the Transport layer is the correct layer where destination port numbers are incorporated into the PDU.