IP Addresses: Everyone Has a Number

Lesson 3 · taught by Pip

Every device that joins a network needs an address. Without one, the network has no way to tell where a message should go, or where it came from. That address is called an IP address — a string of numbers like 192.168.1.42.

Think of a long street where every house has a number painted on the door. A letter carrier doesn't need to know who lives inside; the number is enough to walk up to the right door and drop off the mail. An IP address is that door number for a device.

Two kinds of numbers

Here's the part that surprises people: your laptop usually has two numbers in play, not one.

Inside your home, your router runs a little private street. It hands each device a local address — your laptop, your phone, the smart TV. These local numbers (often starting with 192.168. or 10.) only mean something inside your house. They're like apartment numbers inside one building: useful for the mail room downstairs, meaningless to the rest of the city.

Facing the outside world, your whole home shares one public address, given to your router by your internet provider. That's the number the rest of the internet sees. So to the wider world, every device in your house appears to knock from the same front door.

How a reply finds you

If everyone in the house shares one public door, how does a reply land on your phone and not the TV?

Your router keeps a small notebook. When your phone asks for a web page, the router writes down "phone sent this request" before passing it out the public door. When the answer comes back, the router checks its notebook and forwards it to the exact local number that asked. This bookkeeping is called NAT — Network Address Translation — and it's why one public number can serve a houseful of devices.

So a message finds the right device through a chain of numbers: the public address gets it to your house, and the router's notebook gets it to the right room inside.

One more note: there are two styles of IP address out there. The older style (IPv4) looks like 192.168.1.42 and the world is slowly running out of them. A newer, much longer style (IPv6) was created to make room for the billions of new devices. Both do the same job — naming a spot so messages can arrive.

Your turn

Find your own numbers. On most phones, open Wi-Fi settings, tap your connected network, and look for "IP address" — that's your local number, the one your router assigned. Then, in a web browser, search "what is my IP." The number that comes back is your public one, shared by everything in your home.

Notice they're different. The first is your apartment number; the second is the building's street address. Seeing both makes the whole two-number system click. 🔦

Next we'll look at how you reach a website by name instead of memorizing its number — that's DNS.