Course 01 · Networking from first principles

How Networks Work

Start at the cable in your own house and work outward to the internet. In every lesson you predict what the network will do, drive it yourself, break it on purpose, and check it against real output from a Linux machine. Written for people starting out in IT, and precise enough for a working network engineer to sign off on. Free, no account.

Start Lesson 1-1 → Review & progress

World 1 · 6 lessons · about 6 minutes each

World 1 · Available now

The Local Network

What happens before a packet can even leave your home: addresses, subnets, frames, ARP and switching. Take the lessons in order; each one builds on the last. The tools, games and labs beside each lesson are optional extras.

  1. 1-1 Five Jobs Before a CatStart here

    Five jobs happen before a page loads: DNS, routing, ARP, TCP, and finally HTTP, usually inside TLS. This is the map for the whole course.

  2. 1-2 Who Lives on Your Street?

    An address and its prefix are two separate facts. Read them off a real interface and find your gateway.

  3. 1-3 Slice the Pizza

    A prefix draws a line through 32 bits. Find the network, the broadcast, and the usable hosts without guessing.

  4. 1-4 The Envelope Swap

    Every hop gets a fresh frame with new MAC addresses. The IP packet inside keeps its destination, and each router lowers its TTL by one.

  5. 1-5 Who Has 192.168.1.1?

    ARP asks the whole LAN once, gets one answer back, and caches it.

  6. 1-6 The Switch Learns

    A switch learns where each MAC lives, forwards to one port when it knows, and floods when it doesn't.

Inside a lesson

Six short beats

Each lesson takes about six minutes and follows the same shape, so you always know where you are.

  1. 1 · StoryA three-panel cartoon and one sentence. The whole idea, before any jargon.
  2. 2 · PredictYou guess before you know. Being wrong here is the point: it makes the next part stick.
  3. 3 · PlayA live simulation with one goal, then a short "what just happened".
  4. 4 · TwistOne thing changes. A TTL of 2, a /30, a device that went quiet. Predict again.
  5. 5 · ProveReal output from a real Linux machine, annotated. Then one question with no hints.
  6. 6 · ClearWhat you can now do, a tool to explore, and a Linux lab if you want the real thing.
Coming next · Being written

Worlds 2 and 3

The rest of the course is mapped and being written. Each lesson page already shows what it will cover and opens its interactive tool, so nothing here is a dead end. The tools themselves are all in Labs & Tools.

World 2 · The Web Request

Follow one request from the next hop all the way to the app.

  1. 2-1Routing Tables
  2. 2-2NAT and Ports
  3. 2-3TCP and UDP
  4. 2-4Inside a TCP Connection
  5. 2-5DNS
  6. 2-6HTTP and TLS

World 3 · The Internet

Troubleshoot real traffic. Learn why routes follow policy, not distance.

  1. 3-1Troubleshooting
  2. 3-2Firewalls
  3. 3-3Load Balancers
  4. 3-4Cloud Networking
  5. 3-5How the Internet Routes
  6. 3-6BGP Final level
After a lesson

Make it stick, and show it

Review & progress

Spaced review

Questions from lessons you cleared come back right before you would forget them. Save or move your progress file here too.

 Proof

Your proof page

A shareable page that lists what you can now do, with a "test me" button for anyone who wants to check.

 Reference

Glossary

Every networking term the course uses, explained in context.