Physical and Data Link Layers

Before a packet can cross the internet it has to cross one cable or one radio link, from a laptop to the router in the next room. The bottom two layers do that job: the physical layer turns bits into a signal, and the data link layer wraps each packet in a frame addressed to the next device by its MAC address.

These layers matter because many faults that look like "the network is down" live here: a cable that cannot carry the speed, a switch flooding traffic it should not, a packet too big for a link, or a capture that seems to show the wrong addresses.

The topic starts with the signal and the frame, then shows how a switch learns where to send frames and how ARP finds the MAC address that belongs to an IP address, with an example of a laptop sending its first packet. It closes with the largest frame a link will carry and the addresses that change at every hop. How the next hop is chosen is routing, a later topic.

By the end you can say what a MAC address identifies, predict what a switch does with a frame, and fill in every address on a frame at each hop.