2 Communication and networks
Choose network arrangements and devices by explaining how they move data and share resources.
Mapped to Cambridge International AS & A Level Computer Science 9618, 2027–2029 Version 2. These original lessons and self-checks are independent study support; they are not Cambridge papers, official marks or grades. The course does not yet provide a timed four-paper simulation or a locked-down Paper 4 programming environment.
Before you begin
- Use the 2027–2029 syllabus for your exam year. AS candidates study Sections 1–12; A Level candidates study Sections 1–20. Paper 4 uses a programming language chosen from Python, Java or Visual Basic in console mode.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does DNS help a client find?
What you will learn
- Explain the key representations, structures or processes in this syllabus section
- Apply the method to a new computer-science example
- Justify a design choice and check its limits
Network scale changes the trade-offs
A LAN connects devices in a limited area and is usually managed by one organisation. A WAN spans larger geographic areas and commonly uses links supplied by telecommunications providers. Neither label alone guarantees speed or security.
A switch forwards frames within a LAN using device addresses. A router forwards packets between networks using IP addresses. A wireless access point connects wireless devices to a network; a modem adapts signals for a particular transmission medium.
The internet is a network of networks
A client requests a service; a server provides it. A peer-to-peer arrangement lets participating devices act as both clients and providers. The World Wide Web is one service on the internet, not another name for the whole internet.
A domain name is easier to remember than an IP address. DNS finds the address associated with a domain name so a client can contact the relevant server. Protocols define how communicating systems format and exchange data.
Pause and explain
Which statement best describes the World Wide Web?
Worked example
A school wants classroom devices to connect locally and reach a remote hosted learning platform. Explain the roles of a switch and router.
Show the worked solution
- The switch forwards traffic among devices on the local network.
- The router connects the local network to other networks and forwards packets toward the remote service.
- DNS may translate the platform's domain name to an IP address before communication.
Answer The switch handles local forwarding; the router forwards between the school LAN and external networks.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A message addressed to a server in another network leaves a school's LAN. Which device forwards it between networks, and what address type guides that decision?
Give me a hint
Separate local frame forwarding from inter-network packet forwarding.
Compare my reasoning
- The router connects networks.
- It examines the packet's destination IP address.
- It selects a next network route rather than choosing a local application.
The router forwards it using the destination IP address.
Look for these in your work
- Router identified
- IP address named
- Forwarding role explained
Explain one difference between a client-server network and a peer-to-peer network.
Give me a hint
Compare who provides a resource and how responsibilities are arranged.
Compare my reasoning
- In client-server, a server provides a service to requesting clients.
- In peer-to-peer, participants may both request and provide resources.
- Connect the arrangement to a consequence such as central management or distributed responsibility.
Client-server centralises a service at servers; peer-to-peer distributes service roles among peers.
Look for these in your work
- Both arrangements described
- Roles distinguished
- A relevant consequence stated
A learner enters learn.example into a browser. Describe the role DNS plays before the browser can contact the web server.
Give me a hint
Trace the name-to-address step without claiming DNS sends the page itself.
Compare my reasoning
- The browser needs an address for the named host.
- It queries DNS for the address associated with learn.example.
- The browser can then use the returned address to communicate with the server.
DNS resolves the domain name to an IP address; the browser then contacts the server using that address.
Look for these in your work
- Name lookup described
- Address purpose clear
- DNS separated from page transfer
Common mistakes
- Calling the World Wide Web and the internet the same thing.
- Saying a switch and router perform the same forwarding job.
- Assuming a LAN must be wired or a WAN must be one particular speed.
What can you explain now?
Which device forwards packets between different IP networks?
Compare with the explanation
A router
A router uses network-layer addressing to choose where packets should go.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How do LANs and WANs differ, and how do switches, routers and DNS contribute to communication?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.