Computer Networks Revision — Complete GCSE & A Level Guide (AQA, OCR, Cambridge)

Computer Networks is one of the most mark-dense topics in GCSE and A Level Computer Science. Students who revise it well can pick up marks efficiently because the questions are often very direct — define a term, name a protocol, explain why a technology is used.

This guide covers everything you need for GCSE (AQA, OCR, Cambridge) and introduces the additional A Level content.

Types of networks

LAN vs WAN

Feature	LAN (Local Area Network)	WAN (Wide Area Network)
Coverage	Single site (school, office, home)	Multiple sites, cities, countries
Ownership	Usually owned by one organisation	Typically leased from telecoms providers
Speed	High (up to 10 Gbps internally)	Lower than LAN (varies widely)
Cost	Relatively cheap to set up	Expensive (leased lines, etc.)
Example	School network, home network	The Internet, a bank’s branch network

The Internet is NOT a LAN. It is the world’s largest WAN.

Other network types (A Level + Cambridge)

MAN (Metropolitan Area Network) — covers a city; e.g., a city’s CCTV network
PAN (Personal Area Network) — very short range; Bluetooth devices
SAN (Storage Area Network) — high-speed network connecting storage devices

Network topologies

A network topology describes how devices are connected.

Star topology

All devices connect to a central switch or hub
If one device fails, others are unaffected
If the central switch fails, the whole network goes down
Easy to add/remove devices
Most common in modern LANs

Bus topology

All devices share a single backbone cable
Data travels in both directions along the cable
Simple and cheap to set up
If the backbone fails, the whole network fails
Collision problems — two devices transmitting simultaneously cause data corruption
Now largely obsolete in modern networks

Ring topology

Devices connected in a closed loop
Data travels in one direction
Equal access for all devices
If one device fails, it can disrupt the ring (unless dual-ring)

Mesh topology

Every device connects to every other device (full mesh) or some devices (partial mesh)
Highly resilient — multiple paths for data
Very expensive to implement for large networks
Used in WANs and the Internet’s core infrastructure

Exam tip: For a 4-mark question about star vs bus topology, give two advantages AND two disadvantages of each, not just advantages.

Network hardware

Device	Purpose
Router	Connects different networks; routes packets between them using IP addresses
Switch	Connects devices within a LAN; forwards data to the correct device using MAC addresses
Hub	Broadcasts data to ALL connected devices; older, inefficient; largely replaced by switches
Network Interface Card (NIC)	Hardware in each device; provides a MAC address and physical connection
Wireless Access Point (WAP)	Connects wireless devices to a wired LAN
Modem	Modulates/demodulates signals; converts digital data to analogue signals for transmission over phone lines
Firewall	Monitors and filters incoming/outgoing network traffic based on security rules

Wired vs wireless

Feature	Wired (Ethernet)	Wireless (Wi-Fi)
Speed	Higher (up to 10 Gbps)	Lower (varies, up to ~3 Gbps for Wi-Fi 6)
Reliability	More reliable, no interference	Subject to interference from walls, other devices
Security	More secure (physical access needed to connect)	Less secure (signals can be intercepted)
Flexibility	Less flexible (cables needed)	More flexible (devices can move)
Cost	Higher infrastructure cost	Lower infrastructure cost

How data is transmitted: packets and protocols

Packet switching

Data sent over a network is broken into small chunks called packets. Each packet contains:

Header: source IP address, destination IP address, packet number, TTL (time to live)
Payload: the actual data being sent
Trailer: error checking data (checksum)

Packets may take different routes through the network and are reassembled at the destination.

Advantages of packet switching:

Efficient use of network bandwidth
Resilient — if one route fails, packets take another
Multiple users can share the network simultaneously

Protocols

A protocol is an agreed set of rules for how data is formatted and transmitted.

Why protocols are needed: Different devices (phones, computers, servers) from different manufacturers need to communicate. Protocols ensure they all “speak the same language.”

The TCP/IP model

TCP/IP is the suite of protocols used for communication on the Internet. It has four layers:

Layer	Name	Purpose	Protocols
4	Application	Interface for applications using the network	HTTP, HTTPS, FTP, SMTP, DNS
3	Transport	Breaks data into segments; ensures reliable delivery	TCP, UDP
2	Internet	Addressing and routing packets	IP
1	Network Access	Physical transmission of data	Ethernet, Wi-Fi

Key protocols

HTTP (HyperText Transfer Protocol) — transfers web pages from server to browser. Port 80.

HTTPS (HTTP Secure) — encrypted version of HTTP using TLS/SSL. Port 443. The padlock in your browser.

FTP (File Transfer Protocol) — transfers files between a client and server. Port 21.

SMTP (Simple Mail Transfer Protocol) — sends email. Port 25.

POP3 (Post Office Protocol 3) — retrieves email; downloads and usually deletes from server. Port 110.

IMAP (Internet Message Access Protocol) — retrieves email; keeps email on server (syncs multiple devices). Port 143.

DNS (Domain Name System) — translates domain names (e.g., compscitutoring.com) into IP addresses.

TCP (Transmission Control Protocol) — reliable, connection-oriented. Checks that all packets arrive and requests retransmission if any are lost. Used for HTTP, email.

UDP (User Datagram Protocol) — fast, connectionless. Sends packets without checking delivery. Used for video streaming, gaming (where speed matters more than perfect reliability).

IP addresses and DNS

IP addresses

An IP address is a unique numerical label assigned to each device on a network.

IPv4: 32-bit address, written as four 0–255 values: e.g., 192.168.1.1
IPv6: 128-bit address, written in hexadecimal: e.g., 2001:0db8:85a3::8a2e:0370:7334
IPv6 was introduced because IPv4 ran out of available addresses

A public IP address identifies a network on the Internet. A private IP address identifies a device within a local network.

MAC addresses are hardware addresses assigned to NICs at manufacture. They are 48-bit hexadecimal addresses. MAC addresses identify devices on a local network; IP addresses identify them across the Internet.

DNS (Domain Name System)

DNS is the Internet’s “phone book”. It translates human-readable domain names into IP addresses.

How DNS works:

You type compscitutoring.com into your browser
Your device checks its local DNS cache — if found, done
If not cached, your device asks a DNS resolver (usually provided by your ISP)
The resolver queries the DNS hierarchy until it finds the IP address for that domain
The IP address is returned to your browser, which connects to the server

The Internet and the World Wide Web

Students often confuse these:

The Internet — the global network of interconnected networks (physical infrastructure)
The World Wide Web (WWW) — a service that runs on the Internet; the collection of web pages accessed via HTTP/HTTPS

Other services running on the Internet: email, FTP, VoIP, online gaming.

HTTP vs HTTPS: HTTPS encrypts the data using TLS (Transport Layer Security), so even if packets are intercepted, they cannot be read. All modern websites should use HTTPS.

Cyber security

Common threats

Malware (malicious software) — umbrella term for harmful software:

Virus — attaches to legitimate programs; spreads when the program runs
Worm — self-replicating; spreads across networks without needing a host program
Trojan — disguised as legitimate software; doesn’t self-replicate but creates a backdoor
Ransomware — encrypts the victim’s files and demands payment for the decryption key
Spyware — silently monitors user activity and sends data to attackers
Adware — displays unwanted advertisements; often bundled with free software

Phishing — fraudulent emails or websites that trick users into revealing credentials or downloading malware. Spear phishing is targeted at specific individuals.

SQL injection — attackers insert malicious SQL code into input fields to manipulate a database. Can expose, modify, or delete all data.

Denial of Service (DoS) — overwhelms a server with traffic so it cannot respond to legitimate requests. A Distributed DoS (DDoS) uses thousands of compromised machines (a botnet).

Man-in-the-Middle (MitM) attack — attacker secretly intercepts and potentially alters communication between two parties.

Brute force attack — systematically tries every possible password combination.

Social engineering — manipulates people rather than systems; e.g., pretending to be IT support to get a user’s password.

Countermeasures

Threat	Countermeasures
Malware	Anti-malware software, keeping software updated, not opening unknown attachments
Phishing	User education, email spam filters, multi-factor authentication
SQL injection	Input validation/sanitisation, parameterised queries, principle of least privilege
DoS/DDoS	Firewalls, rate limiting, DDoS mitigation services, CDNs
Weak passwords	Strong password policies, multi-factor authentication (MFA), password managers
Unsecured networks	HTTPS/TLS encryption, VPNs, WPA3 Wi-Fi encryption

Firewalls — monitor incoming and outgoing network traffic and block traffic that doesn’t match security rules. Can be software (on a device) or hardware (on a network).

Encryption — transforms data into an unreadable format. Only someone with the correct key can decrypt it. HTTPS uses asymmetric encryption to exchange a symmetric session key.

Access control — usernames and passwords; principle of least privilege (users only have access to what they need).

Physical security — locks, CCTV, ID badges; often overlooked but critical.

A Level additional content

At A Level, networks topics go significantly deeper:

OSI model (7 layers) — compared to TCP/IP’s 4 layers
Circuit switching vs packet switching in detail
Routing algorithms — how routers decide the best path
TCP handshake — SYN, SYN-ACK, ACK process
Encryption: symmetric vs asymmetric, public/private keys, digital certificates, PKI
Network security: intrusion detection systems (IDS), honeypots, VPNs, DMZ
Compression algorithms in detail
Cloud computing: SaaS, PaaS, IaaS

Key definitions to learn

Before your exam, make sure you can define these terms in one sentence:

Term	Definition
Packet	A small chunk of data with header (addressing info) and payload
Protocol	An agreed set of rules for data communication
IP address	A unique numerical address identifying a device on a network
MAC address	A hardware address permanently assigned to a network interface
DNS	System that translates domain names to IP addresses
Firewall	Software/hardware that monitors and filters network traffic
Encryption	Converting data to an unreadable format using a key
Bandwidth	Maximum amount of data that can be transmitted in a given time
Latency	The delay between sending and receiving data

Networks questions reward students who know precise definitions and can give specific examples. Use the flashcards and question bank on this site for exam-style practice across all three specifications.