How a Web Request Works

The big idea

Sending a request is like mailing a letter:

1. You know the person's name, but you need their address → DNS turns shop.com into an IP address.
2. You establish a reliable delivery route → TCP connection.
3. You put the letter in a sealed envelope → TLS encryption (the "S" in HTTPS).
4. You write the letter in a format everyone understands → HTTP.
5. Someone at the destination reads it and replies → your server code (often asking a database).

The journey of one HTTP request

Step by step

1. DNS: the internet's phone book

Computers talk using IP addresses (142.250.74.46), but humans remember names (google.com). DNS translates them, with caching at every level: the browser, the OS, your router and your ISP.

2. TCP: a reliable connection

TCP guarantees that data arrives complete and in order, resending lost packets. It starts with a three-way handshake:

3. TLS: the sealed envelope

TLS encrypts everything so nobody in between (Wi-Fi owner, ISP) can read or change it. The server proves its identity with a certificate signed by a trusted authority, which is the padlock 🔒 in your address bar.

4. HTTP: the language of the web

An HTTP request is just text:

GET /products/42 HTTP/1.1
Host: shop.com
Accept: application/json
Authorization: Bearer eyJhbGciOi...

And the response:

HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: max-age=60

{ "id": 42, "name": "Keyboard", "price": 49.99 }

HTTP methods

💡 Idempotent = doing it twice has the same effect as doing it once. Deleting order 42 twice still leaves it deleted. Charging a card twice… is not idempotent! (See REST API Design.)

Status codes by family

5. Your server code

Inside the server, a request typically passes through layers:

// A minimal Node.js (Express) endpoint
app.get("/products/:id", async (req, res) => {
  const product = await productService.getById(req.params.id);
  if (!product) return res.status(404).json({ error: "Product not found" });
  res.set("Cache-Control", "public, max-age=60").json(product);
});

HTTP versions at a glance

Where time goes

A typical 200 ms page request might be spent roughly like this:

That's why we use CDNs (serve from nearby), keep-alive connections (skip handshakes), caching and database indexes. All covered in later lessons.

Key takeaways

• DNS finds the address, TCP makes a reliable connection, TLS encrypts it, HTTP carries the message.
• HTTP methods have meanings: GET reads, POST creates, PUT replaces, PATCH updates, DELETE removes.
• 4xx = the client's fault, 5xx = the server's fault.
• Inside the server: middleware → router → controller → service → database.
• Performance work targets each hop: DNS, handshakes, server time, queries and payload size.