Why Networks Exist

Цели урока

• Understand why networks exist: shared resources, communication, resilience, scale
• Know how the internet emerged: ARPANET 1969, RFCs, peering, IXPs
• Tell apart LAN, WAN, MAN, PAN and the typical traffic on each
• See the fundamental ceilings: speed of light, channel physics, latency vs bandwidth
• Use OSI / TCP-IP layering as a mental map for the rest of the course

October 29, 1969, 10:30 PM, UCLA. Charley Kline types "LOGIN" to send to Stanford. "LO" gets through - the network crashes. The first message in internet history: accidental "Lo and behold". Today Cloudflare carries about 20% of all internet traffic on the planet. NYC to Tokyo RTT is roughly 130 ms over fiber, bounded by physics: light travels through glass about a third slower than in vacuum. Networks hit not hardware limits, but physics.

• **FAANG interviews**: "What happens when you type google.com?" - a classic that tests understanding of the entire stack from DNS to HTTP
• **Troubleshooting**: site won't load. DNS? Router? Firewall? Server? BGP outage? Without network knowledge, it's guesswork
• **Cloudflare CDN**: 20% of the planet's traffic, 310+ PoP locations. Latency as a competitive advantage
• **BGP**: the protocol of the internet. 900,000 routes. One config typo and part of the internet goes dark (it's happened)
• **5G**: 10 Gbps throughput. But the ping to Tokyo is still 100 ms. Physics doesn't negotiate - the speed of light sets the floor

What is a network

October 29, 1969, 10:30 PM. Charley Kline types "LOGIN" to send to Stanford. The connection is live. "L" gets through. "O" gets through. "G" - the system crashes. The first message in the history of computer networks: an accidental "LO".

**A computer network** is two or more devices connected to exchange data according to defined rules. It sounds simple. Behind this definition lies all the infrastructure of the modern world - from a home Wi-Fi router to 900,000 BGP routes of the global internet.

The key word is **protocol**. A network doesn't just connect wires. It defines how devices talk: in what order, in what format, what to do on error. Without a protocol, two computers connected by a cable are mute.

A network scales from two laptops with a cable to billions of devices. The principles are the same. Only the complexity changes - and the cost of mistakes.

A brief history of networks

The 1960s. A computer is room-sized and costs millions of dollars. Each one runs in isolation. Moving data between machines means magnetic tapes - physically, by truck. They called it "sneakernet": the network that runs on sneakers.

The idea of connecting computers came from a simple need: scientists wanted to share computing resources without physically hauling tapes. That need became ARPA Network - which grew into everything we have today.

In 50 years, speeds grew by a **billion times**: from 300 bps modem to 100+ Gbps backbone links. But the physical limit hasn't moved: the speed of light in fiber is 200,000 km/s, and the ping from New York to Tokyo will never drop below 70 ms regardless of how much hardware is added.

ARPANET - the ancestor of the internet

October 29, 1969, 10:30 PM. Charley Kline types "LOGIN" for the Stanford IMP. "L" - through. "O" - through. "G" - the system crashes. The first message in the history of computer networks: "LO". An accidental "Lo and behold".

**ARPANET** - a DARPA project, the first **packet-switched** network. An architectural revolution: instead of holding an entire line (like a telephone), data is broken into packets that travel independently, possibly taking different paths.

Myth: ARPANET was built to survive a nuclear strike. Reality: fault tolerance was a consequence of packet switching architecture, not the goal. The main objective was giving scientists remote access to computing resources.

By 1971, ARPANET connected 15 nodes. By 1983, it switched to TCP/IP - the same protocol used today. The military portion split into MILNET. ARPANET became the foundation of the civilian internet - and was officially shut down in 1990. The internet had grown up and could live on its own.

Internet != Web

"Going on the internet" for most people means opening a browser. But the browser shows the **web** - just one application running on top of the internet. Confusing them is like confusing a road with a car. Email works without a browser. Zoom works without a browser. Online games work without a browser.

**The Internet** - since 1983 (TCP/IP transition). **The World Wide Web** - Tim Berners-Lee, 1991, 8 years later. Email existed 20 years before the web. HTTP and HTML came last of all.

Netflix streams video - not web, but internet (QUIC/UDP). Zoom makes calls - not web, but internet (WebRTC). Smart watches sync - not web, but internet. The web is only what opens in a browser over HTTP.

For a network engineer this distinction is a working tool. "Internet is down" means dozens of different causes: cable, DHCP, DNS, BGP, server, firewall. Each diagnosed at its own layer. Confusing internet with web means looking in the wrong place.

Key ideas

• **A network** is 2+ devices connected to exchange data by protocols. From two laptops with a cable to the internet - the same principles
• **ARPANET 1969** - first message "LO". Packet switching instead of circuit switching: data travels independently, network survives node failures
• **Internet != Web**: internet is infrastructure since 1983. The web is one application on top of it since 1991. Email existed 20 years before the browser

What's next

"LO" in 1969. Now it's time to understand HOW it works:

• The OSI Model — Structures network communication into 7 layers
• The Physical Layer — How bits become electrical signals

Вопросы для размышления

• Which devices in a typical home form a network right now? What connects them - protocols or physics?
• If the internet disappeared for a day, which services would be lost besides the web? Email, calls, online games, smart devices?
• Why did packet switching prove more resilient than circuit switching? What happens when a node fails in each model?
• Ping from New York to Tokyo is 100 ms because of the speed of light. How does a CDN work around this physical limit?

Связанные уроки

• net-02-osi-overview — The OSI model is the next step after understanding why networks exist
• os-01-intro — The operating system manages the network stack - an inseparable connection
• db-01-intro — Databases travel over networks - both topics deal with storing and transmitting data
• st-01-feedback-loops — TCP congestion control is a classic example of systemic feedback
• bt-01-overview