Final answer:
The unique identifier at the data link layer for an Ethernet device is the Media Access Control (MAC) address, which is a unique 48-bit number assigned by the manufacturer.
Step-by-step explanation:
The identifier used at the data link layer to uniquely identify an Ethernet device is called a Media Access Control (MAC) address. Each Ethernet device has a unique MAC address encoded by the manufacturer, ensuring that each device has a distinct address on the local network.
This address is a 48-bit number, typically written in hexadecimal form as a series of six pairs of characters (e.g., 00:1A:C2:9B:00:59). It is essential in the process of networking to help devices communicate with each other effectively within the same network segment.