[FredericJ]

MAC addresses are heavily used for routing purposes on a link layer. [Ethernet, WiFi ...]

[csixty4]

^ This. A MAC address is a link-layer network address, just like an IP address. For example, the ARP protocol[1] uses it to identify devices with a given IP address on a network.

[1] http://en.wikipedia.org/wiki/Address_Resolution_Protocol#Exa...