L31 — Computer Networks

🎯Learning Objectives

• Explain what a computer network is and why we need them
• Tell the difference between LAN, WAN, and the Internet
• Identify the three most common network topologies (star, ring, bus)
• Explain the client-server model with an everyday example
• Draw a simple network diagram in draw.io

📖Theory

1. What Is a Computer Network?

A computer network is two or more computers connected to share information and resources.

Without networks:

• You'd carry files on a USB drive between computers
• Every computer would need its own printer
• Email, web browsing, streaming — impossible

With networks:

• Any file on the school server is available at any computer
• One printer shared by 30 computers
• The entire Internet — one giant global network

2. Types of Networks

Networks are classified by their size:

The Internet is not one big network — it's millions of smaller networks (school LANs, company WANs, home networks) connected together by cables under the ocean and satellites in space.

3. Network Topologies

A topology is the pattern of connections between devices in a network. Think of it as the "shape" of the network.

Star topology — all devices connect to a central switch or router.

  PC1   PC2
    \   /
     HUB
    /   \
  PC3   Printer

• Pros: one broken cable only disconnects that one device
• Cons: if the hub breaks, the whole network goes down
• Used in: almost all modern home and office networks

Bus topology — all devices share one cable.

PC1 --- PC2 --- PC3 --- PC4
|_________________________|
        shared cable

• Pros: simple, cheap to install
• Cons: if the cable breaks, everything stops; slow when many devices transmit at once
• Used in: older networks, no longer common

Ring topology — devices form a closed loop.

PC1 → PC2 → PC3
 ↑             ↓
PC4 ← ← ← ← ←

• Pros: predictable performance
• Cons: one broken link breaks the whole ring
• Used in: industrial networks, some fibre networks

4. Wired vs Wireless

Networks transmit data either through cables or through radio waves:

5. The Client-Server Model

Almost everything on the internet follows the client-server model:

• The client is the device that asks for something (your phone, your browser)
• The server is the computer that provides the answer (a website, a video)

Your phone (client)
    |
    | "GET youtube.com/watch?v=abc"
    ↓
YouTube server
    |
    | "Here's the video data"
    ↓
Your phone plays the video

When you open YouTube, your phone sends a request to YouTube's server. The server sends back the response — the video data. Your phone plays it.

The server doesn't know or care whether it's talking to your phone, your laptop, or your smart TV. It just responds to requests. This is why the same website works on all your devices.

6. Bandwidth and Latency

Two important numbers when talking about network speed:

Bandwidth — how much data can flow per second. Like the width of a pipe.

• Measured in Mbps (megabits per second) or Gbps
• Your home internet might be 100 Mbps download / 20 Mbps upload

Latency — how long it takes a packet to travel from client to server and back. Like how long it takes to throw a ball and catch the return.

• Measured in milliseconds (ms)
• Good gaming connection: < 30 ms. Satellite internet: 600+ ms — bad for games.

💻Code Examples

Example A — Pinging a server to measure latency

Open the Command Prompt (press Windows + R, type cmd, press Enter) and type:

You'll see something like:

Reply from 142.250.74.46: bytes=32 time=12ms TTL=116
Reply from 142.250.74.46: bytes=32 time=11ms TTL=116
Reply from 142.250.74.46: bytes=32 time=13ms TTL=116
Reply from 142.250.74.46: bytes=32 time=12ms TTL=116

• time=12ms is the latency — how long it took for a small packet to go to Google and come back
• 12 ms is very good. 200+ ms would feel slow in a game.

Example B — Network diagram in draw.io

A typical school LAN:

  [ Teacher's PC ]
         |
   [ Switch ]
   /   |   \
[PC1][PC2][PC3]   ← student computers
         |
    [ Router ]
         |
   [ Internet ]
         |
   [ School Server ]

We'll draw this in draw.io using network shape icons.

✏️Practice Tasks

⚠️Common Mistakes

Confusing Wi-Fi with the Internet

Wi-Fi is how your device connects to the router (Local Area Network). The router then connects to the Internet through your ISP. If the ISP is down, you still have Wi-Fi — but no Internet.

Confusing bandwidth and speed

Bandwidth is the maximum data rate. Actual speed depends on network load, distance, interference. 100 Mbps internet doesn't mean every download runs at 100 Mbps.

Thinking the server "visits" the client

In the client-server model, the client always initiates. The server waits and responds. The server never spontaneously sends data to a client — unless a connection was established first (like with push notifications).

🎓Instructor Notes

⚡ How to run this lesson (~80 min)

• [5 min] Hook. Ask: how many devices in this room are connected to the school network right now? (Count computers, phones, the printer, maybe a smart board.) That's a LAN.
• [15 min] Theory. Cover types (LAN/WAN/Internet) and topologies. Draw each topology on the board — star first, since it's what they're sitting in.
• [10 min] Client-server demo. Open the browser, go to any site. Open DevTools (F12) → Network tab → reload the page. Show the list of requests the browser made. Each row is one client request → server response.
• [10 min] Ping demo. Run ping google.com on the projector. Discuss the numbers.
• [30 min] Tasks 1 + 2 in class. Task 1 = everyone runs ping in cmd. Task 2 = draw.io network diagram.
• [5 min] Preview L32. IP addresses and DNS — how does the network know where to send data?

💬 Discussion questions

• "When you type 'youtube.com', who is the client and who is the server?"
• "Would you rather have 1000 Mbps bandwidth with 200 ms latency, or 50 Mbps with 5 ms latency? Why? (Depends on the use case!)"
• "Why is the school using a star topology instead of putting all computers on one long cable?"