A2.2.3 · Network architecture
Networking models
By the end you can
Compare client-server and peer-to-peer networks (and hybrids) for control, scalability, security, cost and reliability.
Who does the work in a network? There are two classic answers, and a mix of them.
Client-server#
In the , a server hosts the resources and services, and clients ask for them: request, response. Web browsing, email and online banking all work this way. The server is the centre of control.
- Control: policies, updates and permissions are managed in one place. But an under-resourced server performs badly.
- Security: it is easier to protect one well-kept server than many devices. But a central store of data is an attractive target.
- Reliability: professionally run servers can be very dependable, with backups. But if the server fails, everyone loses the service (a single point of failure).
- Scalability: add more or bigger servers. But that costs money, and heavy demand can overload a server.
- Consistency: one master copy of the data, easy to back up.
Peer-to-peer#
In a network there is no central server. Each peer has equal standing and acts as both client and server, sharing its own resources directly with the others. BitTorrent file sharing, some voice-call systems and cryptocurrencies such as Bitcoin work this way.
- Control: no central authority, so no one can switch it off, but it is hard to enforce rules or trace a problem.
- Cost: no expensive server; the peers share the load.
- Scalability: every new peer adds capacity as well as demand, so popular files get faster to download, not slower.
- Reliability: no single point of failure, but availability depends on peers being online and reliable.
- Security: weaker, because every peer is a potential weak spot and there is no central place to check or protect data.
Client-server one server sends a copy to each person
26.7 minutes
Peer-to-peer people pass pieces to each other
13.3 minutes
Assumes a 1 Gb/s server, home connections that download at 100 Mb/s and upload at 20 Mb/s. Client-server time grows with the crowd because the server is the bottleneck. Peer-to-peer barely changes, because every new peer brings upload capacity of its own.
Hybrid#
Many real systems mix the two to get the benefits of both. A music or messaging service might use servers for accounts, logins and finding other users, while the actual data travels directly between peers. A company may keep its sensitive files on a server but let staff share small files directly. When an exam describes a system and asks which model fits, a hybrid is often a good, well-argued answer.
A bank’s app sends every transaction to the bank’s own computers, which check and record it.
Thousands of people download the same film file, each sending pieces to the others.
A chat app uses its servers to log you in and find friends, but sends voice calls directly between phones.
Three students connect their laptops to swap a project file with no server involved.
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Check yourself
Networking models
Quick questions, made fresh every time and marked here. Do a few until they feel easy.
Exam practice
Paper 1-style questions
5 questions · 19 marks
Write your answer as you would in the exam, then show the mark points and tick each one your answer made. These are our own questions in the style of the exam, not past-paper questions.
1.Outline the difference between the client-server and peer-to-peer models.
[2]Outline: A brief account: the main points, without much detail.
2.Explain one major disadvantage of using a peer-to-peer network for online banking.
[3]Explain: Give reasons: say what happens AND why, or how one thing causes another.
3.Describe two advantages and two disadvantages of the client-server model.
[4]Describe: A detailed account: say what happens, step by step if it is a process.
4.Explain why a peer-to-peer system such as BitTorrent can share a very large file faster than a single server when many people want it.
[4]Explain: Give reasons: say what happens AND why, or how one thing causes another.
5.Compare the client-server and peer-to-peer models for a school of 400 students that wants to share coursework files.
[6]Compare: Similarities and differences, about both things, point by point.
Past papers
Old exam questions on this topic
From HL Paper 1, May 2014 to November 2025. These papers were set on the previous syllabus, so the wording may differ from today's exam, but the topic is the same. Find the questions and markschemes in your school's past-paper pack.
| Paper | Question | Marks | About |
|---|---|---|---|
| November 2017 | 2(a) | 1 | The role of a client |
| November 2015 | 4(b) | 1 | Spotting the clients in a school network |
| May 2014 | 8(a) | 2 | Two features of peer-to-peer systems |
In a nutshell
- Client-server: dedicated servers hold the resources and answer requests from clients. Centralised: easy to control, secure and back up, but a single point of failure and costly to scale.
- Peer-to-peer: every device is both client and server. No central authority, cheap and resilient, and capacity grows with the number of peers, but harder to secure, control and keep consistent.
- A hybrid mixes the two, for example peers sharing files while a central server keeps the index or handles logins.
- Real examples: web browsing, email and online banking are client-server; BitTorrent, some calling and messaging systems and blockchain are peer-to-peer.
- When comparing, cover control, security, reliability, cost and scalability for both models.