What a network is, and why we bother
A network is two or more computers joined together so they can share. The joins can be cables or radio waves (Wi-Fi). That's it.
What a network lets you share
Files. Save homework on the school server. Open it from any room.
Hardware. 30 computers share one printer.
One internet connection for the whole building.
Talking: email, chat, video calls, online games.
Central care. Back up everyone's files and install updates from one place.
What it costs
Money and setup. Cables, switches, servers, and someone to look after them.
Viruses spread. Malware on one machine can reach all of them.
Security risk. More ways in for hackers. Wi-Fi can be picked up outside the walls.
One fault, many victims. If the main server or switch breaks, lots of people stop.
A network = computers joined to share: files, hardware, an internet connection, and messages.
LAN and WAN: small area, wide area
Networks are named by how much ground they cover. A LAN is one site. A WAN joins sites that are far apart.
| LAN (Local Area Network) | WAN (Wide Area Network) | |
|---|---|---|
| Covers | One home, school or office | Cities, countries, the world |
| Owned by | Usually whoever uses it | Often rented from telecoms companies |
| Joined by | Switches, Ethernet cables, Wi-Fi | Routers, fibre, undersea cables, satellites |
| Speed | Usually very fast: few users, short cables | Slower per user: long distances, shared with many |
| Example | The school network | The internet is the biggest WAN |
Why rent instead of build? A school can run cables round its own building. It can't dig a trench from Madrid to London. So it pays a telecoms company to carry its data on lines that already exist.
A PAN (personal area network) is even smaller than a LAN: your phone, earbuds and watch talking over Bluetooth. Nice to know, but not in the GCSE specs.
The internet is a network of networks. The web is one thing on it.
Your home LAN, your school LAN, a mobile phone network and a data centre are all separate networks, with separate owners. Routers join them together. All of those joined-up networks, worldwide, are the internet.
The internet is the roads. The things you do on it are the traffic. The World Wide Web is just one kind of traffic.
| The internet | The World Wide Web | |
|---|---|---|
| What it is | The physical network of networks | A service: linked pages you view in a browser |
| Made of | Cables, routers, satellites, servers | Web pages (HTML) on web servers, joined by links |
| Started | 1969 (ARPANET); the internet protocols from 1983 | 1989–91, Tim Berners-Lee at CERN |
| Analogy | The roads | One kind of vehicle on them (delivery vans) |
"I'll look it up on the internet" is fine in normal life. In an exam, if the question is about web pages and browsers, say the web. If it's about cables and routers, say the internet.
The hardware: what each box does
Your home network has more parts than you think. The white box from your internet company is really four devices in one case. Tap any part of the diagram to see its job.
| Device | Job in one line | Reads which address? |
|---|---|---|
| NIC (network interface card) | Connects one device to the network | Has its own MAC address |
| Switch | Joins devices inside one LAN. Sends data only to the right device | MAC addresses |
| Router | Joins different networks. Forwards packets towards their destination | IP addresses |
| Wireless access point | Lets devices join the LAN by Wi-Fi | MAC addresses |
| Modem / fibre box | Turns data into the signal the outside line uses | None: it just converts signals |
Cables or Wi-Fi?
| Copper Ethernet | Fibre-optic | Wi-Fi | |
|---|---|---|---|
| Data travels as | Electrical pulses | Flashes of light in glass | Radio waves |
| Speed | Fast (often 1 Gbps) | Fastest, over huge distances | Slower; drops through walls |
| Distance | Up to about 100 m | Many kilometres (undersea!) | Tens of metres |
| Interference | A little | None from electricity | Lots: walls, microwaves, neighbours |
| Security | Need to plug in | Very hard to tap | Signal leaks outside: must be encrypted |
| Freedom to move | No | No | Yes |
Why does a switch beat sending to everyone? An old hub copied every message to every socket. Every device had to read and ignore other people's data, and the cable was busy all the time. A switch learns which MAC address is on which socket, so a message only goes where it's needed.
Topologies: the shape of the network
A topology is the layout: what joins to what. It decides what happens when something breaks. The GCSE specs (AQA and OCR) ask for two: star and mesh.
| Star | Mesh | |
|---|---|---|
| Layout | Every device has its own cable to a central switch | Devices join to many others (full mesh: to every other) |
| One cable breaks | Only that device drops off | Data finds another way |
| Centre breaks | Everything stops: a single point of failure | There is no centre |
| Cost | Cheap-ish: one cable per device | Expensive: lots of links |
| Where you see it | Most homes and schools | The internet's routers; mesh Wi-Fi systems |
Cut a cable, break a box
Tap a line to cut that cable. Tap a box to break that device. Who can Your PC still reach?
How many links does a full mesh need? Each of n devices joins to the other n − 1. That counts every link twice (once from each end), so halve it: n × (n − 1) ÷ 2. 5 devices: 5 × 4 ÷ 2 = 10. 30 devices: 30 × 29 ÷ 2 = 435 cables. That's why a whole school is never a full mesh.
Older books also show bus and ring topologies. They are history now and are not in the current AQA or OCR specs. Learn star and mesh well.
Addresses: IP and MAC
Every device on a network needs two kinds of address. One says where it is (IP). One says which card it is (MAC). Like a postal address and a passport number.
IP addresses
An IP address is a device's address on a network. Routers read it to deliver packets.
It can change. Take your laptop from home to school and it gets a new one, like a postal address when you move house. Most are handed out automatically (this is called DHCP).
IPv4 looks like 192.168.1.25: four numbers with dots.
Each number is one byte = 8 bits. From Topic 2: 8 bits make 2⁸ = 256 patterns, so each part runs from 0 to 255. That's why 192.168.300.1 is impossible.
Running out: IPv6
4 bytes = 32 bits. How many different addresses is that? Use the patterns formula from Topic 2.
32 bits make 2³² patterns. Too big to do as 32 twos in your head, so split it up.
Trick: 2¹⁰ = 1,024 ≈ 1,000. Ten bits ≈ a thousand.
32 = 2 + 10 + 10 + 10. So 2³² = 2² × 2¹⁰ × 2¹⁰ × 2¹⁰.
≈ 4 × 1,000 × 1,000 × 1,000 = 4 billion. The exact answer is 4,294,967,296.
The world has over 8 billion people, and many own a phone, a laptop and a games console. Not enough.
So IPv6 uses 128 bits: 2¹²⁸ ≈ 340 undecillion (3.4 × 10³⁸). That's 34 followed by 37 zeros. Plenty for every grain of sand.
IPv6 is written as 8 groups of 4 hex digits: 2001:0db8:85a3:0000:0000:8a2e:0370:7334. Each hex digit = 4 bits, so 8 × 4 × 4 = 128 bits.
MAC addresses
A MAC address is the fixed ID of the network card (NIC) itself, set when it's made. It doesn't change when you move.
48 bits = 6 bytes, written as 12 hex digits: 3C:22:FB:7A:1E:90.
The first half says who made it. The second half is that card's own number.
Switches use MAC addresses to deliver inside a LAN. Routers use IP addresses to deliver between networks.
| IPv4 | IPv6 | MAC | |
|---|---|---|---|
| Size | 32 bits (4 bytes) | 128 bits (16 bytes) | 48 bits (6 bytes) |
| Written as | 4 denary numbers, 0–255 | 8 groups of 4 hex digits | 6 pairs of hex digits |
| Changes? | Yes, depends on the network | Yes | No, fixed to the card |
| Used by | Routers, to deliver between networks | Switches, inside one LAN | |
Addresses starting 192.168. or 10. are private: used inside homes and schools only. Millions of homes all use 192.168.1.x. Your router swaps them for its one public address when packets go out to the internet. That trick kept IPv4 alive.
DNS: the internet's phone book
Routers only understand numbers. People remember names. DNS (the Domain Name System) turns a name like www.bbc.co.uk into an IP address. No single computer knows every name, so it asks a chain of servers.
Follow a DNS lookup
The IP addresses here are made-up example addresses. Real ones change.
| Server | What it knows | Phone-book version |
|---|---|---|
| Resolver | Nothing at first, but it does the asking, and remembers answers (its cache) | A helpful receptionist |
| Root server | Who looks after each ending: .com, .org, .uk, .es | The shelf of phone books, by country |
| TLD server | For its ending, who looks after each name (bbc.co.uk, rae.es) | One country's phone book |
| Authoritative name server | The actual address for each name in its domain | The family's own entry |
Why not one giant list? There are hundreds of millions of domain names, changing every minute, and billions of lookups a second. One server would be overloaded and one fault would break everything. Spreading the job out (and caching answers) keeps it fast and reliable.
Type an IP address straight into a browser bar and it skips DNS. If a website "is down" but its IP address works, the problem was DNS.
Packet switching: chop it up, send, rebuild
A photo is not sent in one piece. It is cut into thousands of small packets. Each packet travels on its own and may take a different route. The receiver puts them back together.
Split. The sender cuts the file into packets and numbers them: 1 of 7, 2 of 7…
Label. Each gets a header: from-address, to-address, packet number. And a checksum at the end.
Send. Each packet goes off on its own.
Route. Every router reads the to-address and passes the packet one hop closer, on the least busy link.
Arrive. Packets turn up at different times, out of order.
Check. The receiver redoes the checksum. If it doesn't match, the packet was damaged.
Rebuild. Put them in order by packet number. Anything missing or damaged gets asked for again.
Route a packet around trouble
Each line shows its delay in milliseconds (ms). The packet takes the fastest total route. Tap a line to cut it. Tap a router to make it busy, then broken.
The checksum, by hand
A checksum is a number worked out from the data. The sender works it out and puts it in the trailer. The receiver works it out again. Here's a very simple version: add up the letter codes, and keep only what's left after dividing by 256 (so it fits in one byte).
The payload is HI!. In ASCII: H = 72, I = 73, ! = 33.
Sender adds them: 72 + 73 + 33 = 178. Less than 256, so the checksum is 178. It goes in the trailer.
On the way, interference flips one bit. I (73 = 01001001) becomes 75 = 01001011, the letter K.
Receiver adds what arrived: 72 + 75 + 33 = 180.
180 ≠ 178. Damaged! The receiver asks for that packet again. Only one small packet is resent, not the whole file.
Why chop it up at all? (1) Lots of people share the same cables at once: small packets take turns, so nobody hogs a line. (2) If a link breaks, packets just go another way. (3) A lost packet means a tiny resend, not starting the whole download again.
A 3 MB photo with about 1,500 bytes per packet: 3,000,000 ÷ 1,500 = 2,000 packets.
Protocols, and the 4-layer TCP/IP model
A protocol is a set of agreed rules for communicating: how to start, what order things go in, how to spot mistakes. Two devices made by different companies can talk because they follow the same protocols.
| Protocol | Stands for | Job | Layer |
|---|---|---|---|
| HTTP | HyperText Transfer Protocol | Fetches web pages | Application |
| HTTPS | HTTP Secure | Same, but encrypted | Application |
| DNS | Domain Name System | Turns names into IP addresses | Application |
| SMTP | Simple Mail Transfer Protocol | Sends email, to and between mail servers | Application |
| IMAP | Internet Message Access Protocol | Collects email; it stays on the server, synced on all your devices | Application |
| POP (POP3) | Post Office Protocol | Collects email; downloads it to one device | Application |
| FTP | File Transfer Protocol | Moves files between computers | Application |
| TCP | Transmission Control Protocol | Splits into packets, numbers them, checks they all arrive, resends | Transport |
| UDP | User Datagram Protocol | Sends packets fast with no checking. Good for games and calls | Transport |
| IP | Internet Protocol | Addresses and routes packets | Internet |
| Ethernet, Wi-Fi | Moves data across one cable or radio link | Link |
Application (purple): the program's own rules. HTTP asks for a page. SMTP sends an email.
Transport (green): TCP chops the data into numbered packets and makes sure they all arrive. Or UDP, if speed matters more than every packet.
Internet (blue): IP adds the from and to addresses, and routers forward packets across networks.
Link (orange): Ethernet or Wi-Fi moves the bits across one physical link, using MAC addresses.
Why layers? Each layer does one job and only talks to the layers next to it. So you can swap one without touching the others. Move from cable to Wi-Fi (link layer): the web page (application layer) doesn't care. Engineers can work on one layer without understanding all four.
AQA 8525 names the four layers: application, transport, internet, link, and which protocol sits in each. OCR J277 asks for the protocols and the idea of layers and why they help, but not the names of the 4 layers. Teach the 4 names anyway: they make it clearer.
Speed: bandwidth and latency are different things
"My internet is slow" can mean two different problems. Bandwidth is how much data can flow per second. Latency is how long each bit of data takes to make the trip.
| Bandwidth | Latency | |
|---|---|---|
| Means | How much data per second | How long data takes to arrive |
| Measured in | Megabits per second (Mbps) | Milliseconds (ms). "Ping" measures it |
| Motorway | Number of lanes | Length of the journey |
| You notice it when | Downloading big files, streaming 4K | Gaming ("lag"), video calls |
| Made worse by | Many users sharing, weak Wi-Fi, old copper cables | Distance, many router hops, satellites far above Earth |
A download time, by hand
A 2.5 GB game update. Your broadband is 50 Mbps. How long, at best?
Same units first. 2.5 GB = 2.5 × 1,000 = 2,500 MB.
Bytes → bits. Speed is in bits, so turn the file into bits too: 2,500 MB × 8 = 20,000 megabits.
Divide by the speed. 20,000 ÷ 50 = 400 seconds.
Make it human. 400 ÷ 60 = 6 remainder 40. So 6 minutes 40 seconds.
Sense check. Real life is a bit slower: other people share the line, and every packet carries a header, not just data.
The race: bandwidth vs latency
Two connections fetch the same thing. Total time = waiting time (latency) + sending time (size × 8 ÷ bandwidth). Which one wins?
Small things (game moves, clicks): latency decides. Big things (films, updates): bandwidth decides.
Client–server vs peer-to-peer
Two ways to organise who does what on a network. Either one computer is in charge (client–server), or every computer is equal (peer-to-peer).
| Client–server | Peer-to-peer (P2P) | |
|---|---|---|
| How | Clients request; a central server responds | Every computer is equal and shares directly |
| Good | Easy to secure, back up and update in one place | Cheap: no server. No single point of failure |
| Bad | Server costs money. If it fails, everyone stops | Hard to secure and back up. Files spread everywhere |
| Examples | School logins, websites, email, Netflix | BitTorrent, some game updates, sharing between two phones |
When you open a web page, your browser is the client. It sends an HTTP request. The web server responds with the page. That is client–server, millions of times a second.
"Server" means a job, not a special kind of box. Any computer that responds to requests is acting as a server. Your laptop can be one.
Common mix-ups
The mistakes nearly everyone makes the first time. Read the left side, then the right.
The internet and the web are the same thing.
The internet is the network. The web is one service on it, like email or games.
The router box at home is just a router.
It's a router, a switch, a wireless access point and a modem in one case.
Switches and routers do the same job.
A switch works inside one LAN using MAC addresses. A router joins different networks using IP addresses.
An IPv4 part can be any number, like 300.
Each part is one byte: 0 to 255 only.
Your IP address is fixed forever, like your MAC address.
IP addresses change with the network you join. The MAC address is the fixed one.
A file travels as one piece along one wire.
It's split into packets that can take different routes and arrive out of order.
Bandwidth and latency both mean "speed", so they're the same.
Bandwidth = how much per second (lanes). Latency = how long the trip takes (distance).
100 Mbps downloads 100 MB per second.
Speeds are in bits: 100 ÷ 8 = 12.5 MB per second at most.
Check yourself
Quick questions first. Then endless practice: a wrong answer shows you the working, step by step.