What is a network? Devices, kinds and measures
Lesson 1 of 4
1What is a network? Devices, kinds and measures
The big pictureWhen you open a site on your phone, your message crosses the home router, then your provider's network, then networks you never heard of until it reaches a server maybe on another continent, and comes back in under a second. That whole path is "a network".
What the picture shows: One packet on its trip: from your private address, through the router that translates it to a public one, across many independent networks to the server, then the answer the same way back.
Map of this part
The concepts in order, click any one to jump to it:
Two ways into the same idea
Think of the post: every house has an address, a letter goes in an envelope with sender and receiver, and post offices pass it from city to city. A network is the same: your device has an address, data is cut into small envelopes called packets, and the "post offices" are devices called routers that pick the road.
A network is nodes and links, wired or wireless, talking through protocols. It is classed by area (PAN, LAN, MAN, WAN), by shape (star, mesh, spine-leaf in data centres) and by model (client-server or P2P). The internet is tens of thousands of autonomous systems (AS) joined by BGP. It is measured by bandwidth (capacity), throughput (what actually gets through), latency (trip time) and loss.
Before You Start
🔌How a computer works
It helps to know that everything in a computer is bits: zeros and ones.
🌍How the internet works (stage 0)
If you read it, this track takes you deeper; if not, this part starts from zero.
The Core Idea
1A network: devices, links and rules
Every network has three things: devices (phone, computer, server, printer), links between them (copper cable, optical fibre, Wi-Fi waves), and rules everyone agrees on, called protocols. Without the rules the signal arrives but nobody understands it, like two people speaking different languages.
The internet is not one network. It is thousands of independent networks of carriers, universities and companies, connected to each other. No one "owns" the internet, and each network decides for itself how to pass packets along.
Networks by area: from the Bluetooth around you to networks joining countries.
2How devices are joined (topology)
The shape decides what happens when a link fails. In a star, every device connects to a centre point (the switch or router), so a cut cable affects only its device. That is the shape of almost every home and office. In a mesh there is more than one road between devices, so it survives failures; critical links and modern home Wi-Fi systems use it.
Four shapes. Bus and ring are nearly extinct today, but exams still ask about them.
3Devices: each works at a layer
A switch joins devices inside the same network and delivers a message only to the right device by its physical MAC address. A router joins different networks and picks the road by IP address. An access point (AP) joins wireless devices to the wired network. A firewall decides what passes and what is blocked.
The "home router" is really four devices in one box: a router, a switch with a few ports, a Wi-Fi access point and a DHCP server handing out addresses. In companies each one is a separate device.
The layer decides what a device reads to make its decision: the signal, the MAC, the IP, or the application.
4Speed is one thing, delay another
Bandwidth is how much data passes per second, like the width of a pipe. Latency is how long the trip takes, like the length of the pipe. Downloading a film needs width; an online game or a video call needs low latency. That is why you can have a 1 Gbps line and still get choppy calls.
Convert it yourself: change the plan or the file size and run.
plan_mbps = 100 # what the provider sells, in megabits per second
file_mb = 700 # a film, in megabytes
speed_mb_per_s = plan_mbps / 8
seconds = file_mb / speed_mb_per_s
print(f"{plan_mbps} Mbps = {speed_mb_per_s} MB/s")
print(f"{file_mb} MB takes about {seconds:.0f} seconds at best")Worked example
The task: draw your home network. The answer: start with the router in the middle (a star). Connect the wired devices (TV, console) and the wireless ones (phones, laptop). One line leaves the router towards the provider over fibre. Each inside device gets an address usually starting 192.168. from the router, and the whole home appears to the world as one public address.
Myth vs Reality
| Myth | Reality |
|---|---|
| The internet and the web are the same thing. | The internet is the roads; the web is one application on them, alongside mail, games and calls. |
| A faster plan fixes every slowdown. | If the problem is latency or weak Wi-Fi, a faster plan changes nothing. |
| Wi-Fi is the internet. | Wi-Fi is only the wireless link inside your home; it can be perfect while the internet behind the router is down. |
Where you meet it at work
🏠IT support
The first question in any fault: is it the device, the local network, or the internet?
☁️Developer
Slow app? The latency to the server may be the cause, not the code.
🏢Network engineer
Picks the shape and devices for what must survive a failure.
Test your understanding
Which device joins different networks and picks the road by IP address?
The router works at the network layer and reads the IP address. The switch stays inside one network and reads the MAC.
Your plan is 200 Mbps. Roughly what is the top download speed in MB/s?
Divide by 8 because a byte is 8 bits: 200 ÷ 8 = 25.
A video call stutters despite a fast plan. Which measure is most likely the problem?
Calls are sensitive to delay and lost packets more than to capacity.
Which shape is most common in home and office networks?
Every device connects to a centre point, and a broken cable affects only one device.
A network inside one company building is a:
A LAN is local to one place; a WAN joins distant places.
Your plan is 500 Mbps but the game "lags". What is most likely?
Speed is how much data, delay is the trip time; games suffer from the second.
Quick review cards
Check Your Understanding
Why does a faster plan not fix a laggy online game?
Because the game suffers from latency, the trip time; a plan adds capacity but does not shorten the road.
Which four devices hide inside a home router?
A router, a switch, a Wi-Fi access point and a DHCP server.
Who owns the internet?
No one alone: it is independent networks, each with an owner, agreeing to exchange packets.
Resources for this lesson
Grouped by type. Start with the official docs for precision, or an article or video for the explanation.
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