For which two purposes does Ethernet use physical (MAC) addresses?

Ethernet uses MAC addresses for two primary purposes. First, MAC addresses uniquely identify devices at Layer 2 of the OSI model. Each network interface card (NIC) has a globally unique MAC address assigned by the manufacturer, ensuring no two devices have the same address. Second, MAC addresses facilitate communication between different devices on the same local network segment. When a device sends data, it uses the destination MAC address to direct the frame to the correct recipient within that local network. Communication with devices on a different network requires Layer 3 (IP) addresses and routing. Differentiating frames from packets is a function of protocol headers, not MAC addresses. Establishing transmission priority is handled by access methods like CSMA/CD, not MAC addresses. Detecting remote devices with unknown physical addresses is a function of protocols like ARP.