Fundamentals of computer networks

A computer network is two or more computers connected so they can share data and resources. Networks underpin almost every digital service we use — the internet, email, cloud storage, online gaming and streaming all rely on networking principles.

Network types (CS5.1)

Type	Scope	Example
LAN (Local Area Network)	Single building or site	School network, home Wi-Fi
WAN (Wide Area Network)	Geographically dispersed	The internet, a bank's branch network

Network models:

• Client–server — dedicated servers provide resources (files, web pages, email); clients request them. Centralised management; easy to apply updates; single point of failure.
• Peer-to-peer (P2P) — each device acts as both client and server. No central server; simple to set up; less secure and harder to manage.

Wired and wireless connectivity (CS5.2)

Medium	Speed	Range	Cost	Reliability
Copper cable (Ethernet)	Up to 10 Gbps	Short–medium	Low	High (no interference)
Fibre optic	Up to 400 Gbps+	Long	Higher	Very high
Wi-Fi	Up to ~10 Gbps (Wi-Fi 6)	~100 m	Low	Moderate (interference)
Bluetooth	~3 Mbps	~10 m	Very low	Short range only

Network topologies (CS5.3)

The topology is the arrangement of devices in a network.

• Star topology — all devices connect to a central switch/hub. Fast, easy to add/remove devices, fault in one device doesn't affect others. Central switch is a single point of failure.
• Mesh topology — every device connects to every other (full mesh) or multiple others (partial mesh). Highly resilient — many paths available; expensive to install.

Protocols (CS5.4)

A protocol is a set of rules for how data is transmitted. Key protocols:

Protocol	Purpose
HTTP / HTTPS	Web pages (S = encrypted with TLS)
FTP	File transfer
SMTP	Sending email
IMAP	Receiving email (stays on server)
TCP	Reliable, ordered delivery of data
UDP	Fast, connectionless delivery (streaming, gaming)
IP	Addressing and routing packets

Network security (CS5.5)

Protecting networks from unauthorised access:

• Encryption — data scrambled so only the intended recipient can read it. Symmetric: same key to encrypt/decrypt. Asymmetric: public key encrypts, private key decrypts.
• Firewalls — software or hardware that inspects incoming/outgoing traffic and blocks unauthorised connections
• MAC address filtering — only devices with approved hardware addresses can connect to the network
• Strong authentication — passwords, 2FA, certificates

The TCP/IP model (CS5.6)

The 4-layer TCP/IP model describes how data travels across a network:

Layer	Role	Example protocols
Application	User-facing services	HTTP, FTP, SMTP, DNS
Transport	End-to-end delivery	TCP, UDP
Network (Internet)	Routing packets	IP
Link (Network Access)	Physical transmission	Ethernet, Wi-Fi

Each layer adds a header when sending (encapsulation) and removes it when receiving (decapsulation).