In this illustration we see a router with a single port, and this port is labeled 88-B2-2F-54-1A-0F. A line extends from the port horizontally to the left and into a link-layer switch. The link-layer switch has three lines that extend to three separate computers arranged vertically. Each computer has a port, and each port has a unique MAC address: the top computer is 1A-23-F9-CD-06-9B, the middle computer is 5C-66-AB-90-75-B1, and the bottom computer is 49-BD-D2-C7-56-2A.