Step 1: Recall what each OSI layer is responsible for. The Physical layer only deals with raw electrical or optical signals, while the Data Link layer is the one responsible for organizing bits into frames and handling addressing for devices on the same local segment. Step 2: See where physical addressing belongs. A MAC address is a hardware level address burned into the network interface card, used purely to identify a device on its immediate local network, not across different networks. Step 3: Match this responsibility to its layer. Because framing and local physical addressing are exactly the Data Link layer's job, and not the Network layer's job which instead uses IP addresses for routing between networks, the MAC address belongs at the Data Link layer. \[ \boxed{\text{Data Link Layer}} \]