Key Idea: A network connects devices so they can exchange data. Everything else — the devices, the protocols, the addressing — exists to make that exchange reliable between machines that were never designed together.
Paper 1
- Naming devices, protocols and topologies.
- Distinguish questions — router vs switch, LAN vs WAN.
Paper 2
- Scenario design: choose a topology or a medium.
- Transfer-time and subnet calculations.
Both
- Selection questions want a justification against the alternative.
- Say what would go wrong with the option you rejected.
🌐 Classifying a network
| LAN | WAN | |
|---|---|---|
| Covers | One site — a building or campus | Towns, countries |
| Who owns the links | The organisation itself | Rented from providers |
| Speed | High | Lower |
| Cost to run | Low | High |
The internet is the global network of networks — the connections themselves. The web is one service running on it, made of pages fetched with HTTP. Email, video calls and file transfer are other services on the same internet.
🔀 Who holds the data
| Client–server | Peer-to-peer | |
|---|---|---|
| Where data lives | Central servers | Spread across every machine |
| Accounts and permissions | Managed in one place | Each machine manages its own |
| Backup | One backup covers everyone | Per machine |
| Suits | Many users — a school, a business | Two or three machines |
📦 The devices, and the question each one answers
Learn the question a device asks about the traffic reaching it, and the device follows.
| Device | The question it answers | Reads |
|---|---|---|
| Switch | Which machine in here? | MAC addresses |
| Router | Which network out there? | IP addresses |
| Firewall | Should this be allowed through at all? | Headers, against rules |
| Access point | How do I get this onto the cable? | Radio ↔ wired signals |
| Modem | How do I put this on the provider's line? | Digital ↔ line signal |
Important: This is the commonest confusion in the unit. A switch works WITHIN one network using hardware addresses; a router works BETWEEN networks using IP addresses. That is why they are different boxes.
📜 Protocols
The ones worth knowing
- TCP — reliable, ordered: numbers segments, acknowledges, retransmits
- IP — addresses and routes packets; no guarantee of arrival or order
- UDP — no acknowledgement; suits live audio and video, where a retransmission arrives too late
- HTTP / HTTPS — fetching pages; HTTPS adds encryption and a certificate proving identity
- DNS — translates a domain name into an IP address
- SMTP sends email; POP or IMAP retrieves it
IP gets packets across any network but may lose or reorder them — keeping it simple is what lets it run over anything. TCP sits on top and adds the guarantees applications need, so the application sees a reliable stream over an unreliable network.
📝 Exam-style questions
A 600 MB software update is downloaded over a connection with a usable rate of 25 Mbps. Determine the time taken, in minutes. Take 1 MB as 8 megabits.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A veterinary practice with one site and twelve computers is choosing between a client–server and a peer-to-peer network. Suggest which it should use, justifying your choice.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A video call and a file download run over the same connection. Explain why the video call uses UDP while the download uses TCP.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
✅ Quick check
Cover the answers.
Switch or router — which reads IP addresses? The router. A switch reads MAC addresses within one network.
What does DNS actually do? Translates a domain name into an IP address. It does not store pages.
Why does HTTPS need a certificate as well as encryption? Encryption hides the traffic; the certificate proves who is at the other end. Without it you could be encrypting perfectly to an impostor.
Bandwidth and latency — are they the same thing? No. Bandwidth is the rate; latency is the delay. A satellite link has high bandwidth and terrible latency.
Which protocol sends email, and which retrieves it? SMTP sends; POP or IMAP retrieves.
Exam tips
- Router = between networks, IP addresses. Switch = within one network, MAC addresses.
- A rate in Mbps is megaBITS — convert a file size in MB by multiplying by 8.
- Name the protocol AND its purpose; a bare list of acronyms scores little.
- TCP guarantees delivery and order; IP guarantees neither; UDP does not even try.
- For a selection question, say what would go wrong with the option you rejected.
- The internet is the network; the web is one service running on it.