MAC is Media Access Control. A MAC address is relevant in Multi-Access networks like Ethernet where you have several hosts on the same network. In such a network, you have to do an ARP broadcast to find the MAC address of the host, so you can send packets address to it and it only.
A serial line has only two ends, so it is fairly obvious where to send the packets. That is why a serial line doesn't have a MAC address. Usually. Or does it?
HTH
Sandy
