MAC addresses are like a permanent IP address for every network device that is made. It is stored in EPROM (Eraseable and Programmable Read Only Memory) they only really started being used effectively in the first days of switches as opposed to hubs. I think we all probably know what a hub is, It's that thing we plug into at LAN parties. hubs communicate with computers by broadcasting requested information to ALL network devices that are connected to it. (That is how a packet sniffer works, It gathers this broadcasted info as the hub is putting it out) Switches, on the other hand, keep a table of all MAC addresses connected to it and which port they're connected to, therefore switches get the requested data and send it ONLY to the computer or other device that requested it.

Modems are no different than network cards other than they are slower because they work on a single-pair long-distance line that is often sent over t1 repeaters (this is what makes them slow). when your modem connects to an ISP you are actually calling another modem on the other end of the line. The group of modems available is called a modem pool. The other modem's serial port connects it to a router which acts like a switch that keeps a table with your MAC address and which modem in the pool your data should go through. When the router sends information from your PC, It also accompanies it with your IP address and MAC address and other information such as IP port and client name.

Any server that you connect to acts as a switch also that keeps your IP and MAC addresses in a table to send your data to you. If the server blocks your MAC address, you are no longer able to access said server. Each router and switch and modem has a MAC address as well, so if a server blocked your ROUTER's MAC address then all computers and devices connected to it would likewise be blocked.

Except the hardware is excellent ... the software is ... lacking, to put it softly.