Key Idea: Topology is how the devices are physically connected. A networking model is how the software is layered. Both exist to make a large network manageable — and every topology question is a trade-off, because no shape wins on everything.
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.
🕸️ The topologies
| Star | Mesh | Bus | Ring | |
|---|---|---|---|---|
| Wiring | Each device to a central switch | Many links between devices | One shared backbone | A closed loop |
| One cable fails | That device only | Routed around | Splits the network | Breaks the loop |
| Simultaneous traffic | Yes, full speed | Yes | No — one at a time | Passes around |
| Cost | A cable per device | Highest | Lowest | Low |
n devices fully meshed need n(n − 1)/2 links: 8 devices need 28, and adding a ninth adds 8 more. That growth is why mesh is used between a few important sites, never to every desk.
Important: Reliability · speed · scalability · collisions · cost. No shape wins on all five, so the answer is always a trade-off — and the marks are in naming which ones matter here.
Client-server or peer-to-peer
Client-server: A central **server** holds the data; **clients** ask it for what they need. Easy to back up, secure and update in one place. The server is a **single point of failure** and costs money to run. Web browsing, email, online banking.
Peer-to-peer: Every machine (**peer**) is equal and can both ask and serve. No central server to buy; capacity grows as peers join. Security and backups are left to each peer. File sharing, some VoIP, blockchains.
✂️ Segmentation
Two reasons, always
- Security — a compromise is contained where it started, instead of reaching everything
- Performance — broadcasts stay inside their segment, and capacity is shared among fewer devices
- A VLAN groups devices logically, regardless of where they are plugged in
- A subnet mask marks which part of an address is the network and which the host
h host bits give 2ʰ − 2 usable addresses. The all-zeros address names the network and the all-ones address is the broadcast, so neither can be given to a device.
📝 Exam-style questions
A company fully meshes its 6 regional offices. Determine how many links this needs, and how many more are needed when a seventh office opens.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A hospital is wiring a new wing: 40 computers on one floor, plus a resilient link back to the main building. Suggest a topology for each, justifying both choices.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A subnet mask of 255.255.255.0 is used. Determine whether 172.20.7.15 and 172.20.9.15 are on the same subnet, and what this means for traffic between them.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
✅ Quick check
Cover the answers.
How many links does a full mesh of 10 devices need? 10 × 9 ÷ 2 = 45.
In a star, what happens if the central switch fails? Every device loses connectivity — it is the single point of failure.
A subnet has 8 host bits. How many usable addresses? 2⁸ − 2 = 254. Two are reserved: the network address and the broadcast address.
Give the two reasons a network is segmented. Security (a compromise is contained) and performance (broadcasts stay local).
Exam tips
- Full mesh needs n(n − 1)/2 links — quote the formula, then substitute.
- Name which of the five criteria matter in THIS scenario rather than listing all five.
- Usable addresses = 2ʰ − 2. Subtracting the two reserved addresses is the step that gets missed.
- Compare only the masked octets when deciding whether two addresses share a subnet.
- Segmentation contains a breach; it does not prevent one.