So far we've talked about how IPv4 encodes data into a packet, and how routers learn which direction to forward those IPv4 packets based on the destination IP address and the route table.
But in the end, routers and hosts need to encode the IPv4 packet onto a physical medium. Examples of physical mediums include fiber, twisted pair, coax, radio waves, lasers, and microwaves. Each encoding rate and medium requires a specification or protocol definition.
Ethernet is a family of similar encoding specifications which is dominant on the Internet today. Ethernet uses a 48-bit address known as a MAC address or hardware address. There are ethernet specifications for speeds ranging from 10 megabit to 10 gigabit and for various copper and fiber physical media.
Using ethernet requires the ability to map an IPv4 address into a MAC address. That is accomplished using the address resolution protocol (ARP).
In this episode, we introduce the ethernet frame format, ethernet address, and how ARP works.