Every Device Has an Address

Unit 1 left a mystery sitting on every packet: the address label. A parcel with no address is trash; a parcel with a good one can cross the planet through twenty networks that have never heard of you. So Unit 2 opens where any postal system must — with the addresses themselves.

The IP address

Every device connected to a network holds an IP address — a numeric address it can be reached at. (IP stands for Internet Protocol, the shared rulebook for addressing and delivering packets; Unit 3 has more to say about rulebooks.) Here's one, up close:

203.0.113.7

Four numbers, dots between, each between 0 and 255. That's the classic format, called IPv4 — and reading one is like reading a mailing address that narrows as it goes: the early numbers say roughly which network in the world (the way a country and city do), the later ones pin down which device within it (the street and door). Routers lean hard on that narrowing, as you'll see later in this unit: the label itself hints at which direction to pass the parcel.

Two truths to install early, because both fight intuition:

• Addresses are assigned, not tattooed. Your phone doesn't own an IP address the way it owns a serial number. It's given one when it joins a network — at home one address, at school a different one, at the café a third. Same phone, three addresses in a day. (Who does the giving is the very next lesson.)
• Every device has one — including yours, right now. Addresses aren't a server thing. The phone in your hand holds an IP address this second; the reply packets carrying this page found you by it. Somewhere in your settings screen it's listed, quietly doing its job.

Running out of numbers

Now do some arithmetic the internet's founders did too late. Four numbers, 0–255 each: about 4.3 billion possible IPv4 addresses. In the 1980s that looked like infinity. Then humanity connected its phones, laptops, TVs, cars, doorbells, and light bulbs — billions of devices — and the unthinkable happened: we ran out of addresses. Genuinely. New IPv4 addresses are essentially gone, traded and rationed like scarce real estate.

The long-term fix is IPv6: the same idea with vastly longer addresses — written in a hex format like 2001:db8::7, with room for 340 undecillion devices (a 39-digit number; every grain of sand on earth could host a trillion internets). Same narrowing structure, same role on the parcel label, no scarcity, ever again. The two systems run side by side today, and your phone likely holds one of each. Where this course shows addresses, it uses friendly IPv4 — the idea is identical.

But notice: IPv6 rollout has taken decades, and the IPv4 internet didn't collapse while waiting. Billions of devices, four billion addresses... the math shouldn't survive. It survives because of a clever, everywhere-at-once workaround happening in your own home right now — one address doing the work of many. That trick is two lessons away. First: the receptionist who hands your phone its address every time you walk in the door.