Which layer adds the source and destination MAC addresses to data and forms frames?

• A. Layer 2 Data Link
• B. Layer 1 Physical
• C. Layer 3 Network
• D. Layer 4 Transport

Answer: (A)

MAC addressing and framing happen at the Data Link layer. When a packet from the Network layer is prepared for transmission, the Data Link layer encapsulates it into a frame by adding a header that includes the source and destination MAC addresses and a trailer for error detection. This creates a locally deliverable unit that can be sent over the physical medium, with switches using those MAC addresses to forward the frame to the correct device. The Physical layer only handles sending the raw bits, not addressing. The Network layer adds IP addresses for routing between networks, and the Transport layer deals with ports and end-to-end communication. So, the Data Link layer is responsible for adding MAC addresses and forming frames.