What happens when you open a web page

You type an address into your browser, press Enter, and within a second a page appears. Behind that second is a chain of steps involving several protocols and many devices. Understanding this chain is the foundation of networking, and it gives you a map for the rest of the course.

The journey in ten steps

1. Your computer already has an IP address, usually given to it by your router through DHCP when it joined the network.
2. The browser needs the server's IP address, so it asks DNS to translate the name, such as example.com, into an address.
3. Your computer checks whether the server is on the local network. It is not, so the request must go to the default gateway, your router.
4. Your computer finds the router's hardware address using ARP, so it can send it a frame on the local network.
5. The router forwards the packet toward the internet. On the way out, it replaces your private address with its public one using NAT.
6. Routers across the internet pass the packet along, each choosing the next hop based on its routing table, until it reaches the server's network.
7. The browser and server set up a TCP connection with a three way handshake (or use QUIC over UDP for HTTP/3).
8. They negotiate encryption with TLS, and the browser checks the server's certificate.
9. The browser sends an HTTP request, and the server replies with HTML, followed by more requests for images, styles and scripts.
10. Responses travel back along a similar path, the router reverses its NAT translation, and your browser renders the page.

Every one of these steps can fail, and each failure looks different. A DNS problem gives a "server not found" error. A routing problem might give timeouts. A certificate problem gives a security warning. Knowing the chain lets you guess where to look.

Try it yourself

Open a terminal and run:

ping -c 4 example.com

On Windows, use ping example.com. You will see the IP address DNS returned and the round trip time for each reply. Then run:

traceroute example.com

On Windows the command is tracert example.com. You will see a list of routers between you and the destination. Some may show asterisks, which simply means they do not reply to these probes.

How the course follows the journey

Section 1 covers the big ideas: layers, addresses and ports. Section 2 looks at your local network, section 3 at crossing the internet, section 4 at DNS and the web, and section 5 brings it together with troubleshooting.