Networking and the Internet, from First Principles

Have you ever wondered what happens when we text, call, or video chat with a friend or a colleague on another continent, and their reply arrives in a fraction of a second, as though they were in the same room? Behind the scenes, a chain of invisible conversions takes place: your voice, video, or message is translated into radio waves crossing the room to your Wi-Fi router, then electrical pulses in copper (or light, if you have a fiber connection), and then flashes of light inside a glass strand thinner than a hair lying deep on the ocean floor, only for the entire sequence to play in reverse at the other end. I find it mind-boggling that we can communicate instantly with anyone in the world by doing nothing more than creating controlled, patterned disturbances of electricity, light, and radio.

The message passes through equipment owned by dozens of independent companies in different countries. None of them coordinated with the others specifically for this message transfer, and none of them knows the full path your data took, they just hand it off to the next closest route. There is no central computer directing the traffic, and no single company owns the internet infrastructure. Yet it works, billions of times every second, so reliably that we only notice it when a call stutters or video buffers.

The software article followed the story of a single machine, from electrons in silicon up to the software you run. This article follows the story of the connections between those machines. Like the layers of computing, the internet was not designed in one stroke; it accumulated over decades, and each protocol makes sense only once you see the concrete limitation it was invented to fix. It is easy to mistake the result for something engineered to a finished blueprint, because failures are rare enough to feel like the system was always this reliable. In reality, every mechanism in this article, packet switching, TCP, DNS, and TLS, was a patch for a specific problem, deployed decades after the internet already “worked”, and the pressure that produced them hasn’t stopped: it now comes from new physical links, new failure scenarios, and new demands from software that didn’t exist when the layer beneath it was designed.

My aim is to build this understanding from first principles. By the end, many of the everyday mysteries of using the internet will make intuitive sense under a single, coherent mental model: how the padlock in your address bar protects your credit card details, whether a dead page is the website’s fault or a failure at your own end, why a webpage can feel sluggish even on a “gigabit” connection, and how your data dynamically reroutes around a failing undersea cable half a world away.

We Were Sending Bits Even Before Computers Existed

Networking is much older than computing, and older than electricity too. The word network itself originally meant exactly what it sounds like, a net-like fabric of threads or cords crossing at regular intervals. In the early 19th century, engineers borrowed the term to describe interconnected transit routes like canals and railways. When the electrical telegraph arrived in the 1840s, the word drifted naturally to describe the systems of wires and stations that carried its signals.

Yet the basic physical principle of a network link remains the same as the simplest mechanical connection. Knot a string tight between two tin cans, speak into one, and the string carries the vibration of your voice to the other as mechanical motion with no amplifier or relay, just a wave losing energy to friction and slack with every meter it crosses. That is already the whole principle behind every link built since, vary a physical quantity at one end and measure it at the other. What the string can’t do is carry a signal any real distance without it dying in the line. The telegraph’s true breakthrough wasn’t just replacing string with electrical wire, but overcoming this physical limit of distance.

In 1844, Samuel Morse sent the message “What hath God wrought” from Washington to Baltimore over a copper wire, using Morse code, a system of short and long electrical pulses. Notice what the telegraph actually was, a digital network. It did not transmit the sound of a voice; it transmitted discrete symbols from a fixed alphabet. That choice had an advantage the Victorians understood well. An electromechanical relay along the line didn’t need to pass the wave itself; it only needed to detect whether a pulse was present, and then recreate a brand new, clean copy of that pulse to send down the next segment of wire. Discrete symbols plus regeneration meant a message could cross a continent without degrading, something no analog signal could do.

Notice also what had to exist before the wire could carry anything, an agreement between sender and receiver. The telegraph only worked because both ends held the same table in advance, which pulses stood for which letters, and how operators signaled “received” or “repeat.” This shared rulebook is a protocol. Every protocol in this article (IP, TCP, DNS, TLS) is the same, a published agreement on message formats and who says what when, allowing independent machines to communicate with each other.

The simulator below sends Morse’s message down that historic line. Watch the pulses fade and pick up noise along each span of wire, and what the relays do about it, then switch the relays to bare amplification and see why the discrete alphabet (which modern computing simplified even further into binary bits) was such a smart choice.

Morse pulses fade and pick up noise along every span of wire. Because the network transmits discrete symbols, a relay doesn't need to pass the wave itself; it only needs to detect whether a pulse was present, and recreate a brand new, clean copy of that pulse. Switch the relays to bare amplification and the noise of each span rides into the next, until Baltimore misreads the message. Discrete symbols plus regeneration is why a message could cross a continent without degrading, something no analog signal could do.

The telegraph network even solved routing, with people. A message from a small town to another small town passed through relay offices, where operators received it, punched it onto paper tape, and retransmitted it down whichever outgoing line led closer to the destination when that line became free. Messages queued in bins during busy hours. Hold onto this idea, a hundred years later we will rebuild it with electronics and call it a router.

Morse’s own line only had to cross one state. Crossing an ocean took longer, requiring a decade of costly setbacks and a painful education in the physics of underwater cables. Cyrus Field’s first transatlantic telegraph cable went live in August 1858, carrying a congratulatory exchange between Queen Victoria and President Buchanan; three weeks later it was dead, its insulation damaged in handling and, some think, finished off by an engineer’s overvoltage trying to push a signal through it. The successful cable came in 1866, laid by the SS Great Eastern, at the time the largest ship ever built and the only one that could carry the roughly 4,000 kilometers of cable in a single piece. The ocean floor has carried communication cables ever since, a story we will return to when telegraph wires evolve into coaxial copper and, eventually, glass fiber.

The underlying trick generalizes to every link ever built since. To move bits between two points, you vary some physical quantity at one end and measure it at the other, on an agreed schedule. A bit, short for “binary digit,” is the smallest possible piece of information there is, a single choice between exactly two states, conventionally written 1 or 0, and everything this article measures, sends, or stores is ultimately some number of these (the software article builds this up from transistors and logic gates, if you want to understand that too). Group eight of