At which OSI layer is a destination IP address added to a PDU during the encapsulation process?

The correct answer is that a destination IP address is added to a Protocol Data Unit (PDU) specifically at the Network layer of the OSI model. This layer is responsible for determining how data is sent across the network and includes the routing of packets of data.

When a data segment is encapsulated into a packet at the Network layer, an IP header is attached, which contains important control information such as source and destination IP addresses. The presence of these addresses is crucial for directing the packet through various networks and ensuring it reaches the correct endpoint.

In contrast, the other layers focus on different processes: the Transport layer is primarily responsible for end-to-end communication and flow control, the Data Link layer manages node-to-node data transfer and error detection, and the Application layer deals with user interface and application-level data. Therefore, the encapsulation of the destination IP address occurs at the Network layer, reinforcing its critical role in network communication and packet routing.