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Level 3: The Software World
Lesson 4 · +10 XP

The internet, basically

You’ve probably used the internet for twenty years without ever asking what it is. Let’s fix that in two minutes.

The internet is a giant network of computers that all agreed to use the same set of rules to talk to each other. A URL is the address you use to point at a specific program on one of those computers.

That’s most of it. Everything else is detail.

A URL is a postal address

Look at this URL:

https://claude.ai/chat

Three pieces, like an envelope:

  • https — the protocol. Which set of rules the two computers are agreeing to use. (https is “secure web traffic.” There are others — ftp, ssh, wss — but you’ll mostly see https.)
  • claude.ai — the domain. Which computer (well, which group of computers) you’re talking to. Like the building name.
  • /chat — the path. Which specific page or thing inside that building. Like the apartment number.

When you type that URL and hit Enter, your browser (a client) sends a request to a server somewhere, addressed to /chat at claude.ai, using the https rules. The server answers. Page loads.

The thing that trips people up

You won’t see the IP address. The URL doesn’t look like a postal address — it doesn’t have numbers and a zip code.

Underneath, there is a number. Every computer on the internet has an IP address that looks like 142.250.190.46. But nobody wants to type that, so we have a phonebook — the DNS — that converts claude.ai into the right IP automatically.

You don’t need to remember any of this. Just hold this: a domain name is a human-friendly nickname for a computer’s actual address.

localhost — your own computer’s address

There’s one URL worth knowing now:

http://localhost:4321

localhost means “this computer, right here, the one I’m on.” When you run a website’s code on your own laptop (which you’ll start doing in Level 5), it usually shows up at a URL like that. Same internet rules, but the conversation never leaves your machine.

The :4321 is a port — like the apartment number on your own computer. A computer can run dozens of programs at once, each listening on a different port. Most websites use port 80 or 443; local dev servers use whatever’s free.

What you actually need to remember

Out of all of that:

  • A URL is an address.
  • The domain is which computer.
  • The path is which page on that computer.
  • localhost is you, right here.

The rest is trivia until you need it.

What’s next

Now that we know how the request gets to the server, let’s look at the shape of the conversation: the contract that says what the client can ask and what the server promises back. It’s called an API.