At which OSI layer is a source MAC address added to a PDU during the encapsulation process?

The source MAC address is added to a PDU (Protocol Data Unit) at the Data Link layer of the OSI model during the encapsulation process. In this layer, frames are created, which include the MAC (Media Access Control) addresses of both the source and the destination devices. This addressing is crucial for local network communication, allowing devices on the same local network segment to identify and communicate with each other. The Data Link layer is responsible for node-to-node data transfer and handles the physical addressing necessary for this purpose.

In contrast, the other layers mentioned have different roles. The Network layer is involved with logical addressing and routing of packets across multiple networks, while the Transport layer focuses on end-to-end communication and data flow control between hosts, utilizing ports for multiplexing. The Application layer, on the other hand, is closest to the end-user and deals with high-level protocols like HTTP and FTP, which do not concern themselves with MAC addressing. Thus, the correct understanding of where MAC addresses are utilized clearly points to the Data Link layer.