Before you can understand a single sentence of the parent note, you need to be fluent in a handful of words and symbols it throws around: IP address, MAC address, frame, broadcast, unicast, LAN / broadcast domain, cache, EtherType, and the two "layers." This page builds each one from nothing, in the order they depend on each other.
Every device on a network has two completely different addresses. This is the single most important idea, so we start here.
The picture below shows both names living on the same laptop at once. Look at how the MAC (red) is stuck to the card while the IP (blue) is a sticker the network hands out.
Why the topic needs this: ARP is literally defined as the translator between these two. If you don't hold both in your head at once, the phrase "resolve IP → MAC" is meaningless. See Ethernet & MAC addressing and IP addressing & subnetting for the deep dives on each name.
The parent note keeps saying Layer 3 and Layer 2. These are floors in a stack of software, each handing work down to the one below.
A frame is like an envelope; a packet is the letter inside that envelope. When you want to send data, Layer 3 writes the letter (with IP addresses on it) and hands it down to Layer 2, which stuffs it in an envelope and writes MAC addresses on the outside.
Why the topic needs this: the whole point of ARP is that it sits between these two layers, feeding Layer 2 the MAC that Layer 3 could not supply. (Runs directly over the Ethernet part of Layer 2 — that's why the parent calls it "a Layer-2 protocol.")
Now zoom into that envelope. An Ethernet frame has, on its outside, a few labelled fields. The parent note uses three of them.
The 0x prefix just means "the following digits are hexadecimal (base-16)," a compact way to write raw bytes. 0x0806 is simply the number 2054, but nobody cares about the decimal — it's a fixed label the receiver matches against.
Why the topic needs this: the parent states ARP "runs directly over Ethernet, EtherType 0x0806." Now you know what that string of symbols actually selects.
Picture a room with walls (the router). Shouting fills the room but stops at the walls. ARP is shouting, so ARP only works inside one room.
Why the topic needs this: this is exactly why the parent's biggest "common mistake" is "ARP works across the internet." It doesn't — the walls of the broadcast domain are ARP's world.
Why the topic needs this: the parent's "ARP cache" section, the dynamic-vs-static distinction, and why gratuitous ARP is needed for fast failover all rest on understanding timeout.
Read it top-down: the two names feed the two layers; the layers give you frames with fields; fields give you broadcast vs unicast; frames live inside a broadcast domain; and a cache stores the results — all of it converging on ARP.
