Computer systems and networks · GCSE Computer Science
Networks
LANs and WANs, hardware, topologies, protocols, DNS and the factors that make a network fast, reliable or neither.
Rules (protocols) matter more than wires.
The important bits
What you need to know
- 1
A LAN covers a small geographic area you own (school, office). A WAN covers a large area, often hired from telecoms (the internet is the WAN of WANs). The distinction is ownership and span, not “Wi-Fi versus cables”.
- 2
Star topology: devices link to a central switch. One cable fails, one device drops; the switch is a single point of failure. Mesh: many routes, resilient, expensive. Bus is historical GCSE knowledge on some specs.
- 3
Hardware: NIC (addresses the host), switch (forwards frames inside a LAN using MAC addresses), router (forwards packets between networks using IP), WAP (wireless access), transmission media (copper, fibre, radio).
- 4
MAC addresses identify a network interface on the local link; IP addresses identify a host on an internetwork and can change. You need both ideas: Ethernet delivers by MAC; the internet routes by IP.
- 5
DNS maps human names (revisiongems.com) to IP addresses so people do not memorise 32-bit numbers. If DNS fails, the site can be up and still unreachable by name.
- 6
Protocols are agreed rules. TCP provides reliable, ordered streams (retransmit on loss); IP routes packets; HTTP/HTTPS request web pages (HTTPS adds TLS encryption); FTP moves files; SMTP/IMAP/POP move email. Layers stop one huge rulebook.
- 7
The TCP/IP 4-layer model (application, transport, internet, link) is the usual GCSE stack. Know which protocols sit where, not a Cisco-level OSI rant unless your board asks.
- 8
Performance factors: bandwidth, latency, number of users, errors/interference, wired vs wireless, and hardware bottlenecks. Encryption protects confidentiality; it does not by itself make a network faster.
Go deeper
Switch versus router, in one picture
A switch is a LAN device: it learns which MAC address lives on which port and forwards Ethernet frames accordingly, so traffic to the printer need not flood every room. A router sits at a network boundary: it inspects IP addresses and chooses the next hop toward another network, applying NAT and firewall rules in a home box. Calling the home “router” a router is fair; it is also a switch and a WAP in one plastic shell. In exams, separate the jobs. If the question is “how does data find a host on the same LAN?”, think switch and MAC. If it is “how does it reach a server in another country?”, think IP, routers, and DNS to find the IP in the first place.
Go deeper
Why HTTPS is HTTP plus a lock, not a different web
HTTP is the request/response language of the web: GET this path, here is HTML. Anyone on the path can read it. HTTPS wraps that language in TLS: the browser checks a certificate, then encrypts the payload so a café Wi-Fi snoop sees ciphertext. It does not make the site morally better or faster; it makes eavesdropping and some tampering harder. Mixed-content pages (HTTPS page loading HTTP images) punch holes in that story. For 3-mark security questions, pair HTTPS with other controls — it is not a complete cyber-security policy, just the transport confidentiality piece.
See the idea in action
Explain what happens when you type a URL. Browser asks the resolver (DNS) for the IP. A packet is built: HTTP(S) request, TCP ports, IP destination, frame with the gateway’s MAC. Routers hop the packet across the WAN. The server’s TCP stack reassembles, the web server answers, TLS if HTTPS. If you skip DNS or confuse MAC with IP, the story breaks. That sequence is a 6-mark gift if you keep the order.
Exam technique
Turn knowledge into marks
Define protocol as “a set of rules for communication” then name one. Empty “rules” without an example is a half-mark habit.
Common mistakes
Do not give these marks away
- 01
Saying a router and a switch do the same job.
- 02
Claiming the internet is a LAN, or that Wi-Fi is a topology rather than a medium.
- 03
Treating DNS as a search engine. It looks up names; it does not rank pages.
What is the main job of DNS?
ATo encrypt all packets on a LAN
BTo translate domain names into IP addresses
CTo assign MAC addresses to switches
DTo make star topologies more reliable than mesh
Show the answer
To translate domain names into IP addresses. People use names; routers use IP addresses. DNS is the directory. Without it, you would have to know the numeric address of every service.
Quick questions
If this is the bit you searched
Is fibre always better than copper?
Fibre has higher bandwidth, longer distance and immunity to electrical interference, but it is dearer to terminate and less forgiving to bend. Copper is still fine for short cheap runs. Pick the factor the question asked for.
What does a firewall actually do?
It allows or blocks traffic using rules (ports, IPs, sometimes application type). It is not antivirus. A misconfigured firewall is either a brick wall or an open door.