Networks

Network Types

A computer network is a collection of devices connected together to share resources and communicate. Networks are classified by their geographic scale and ownership.

Network Type Overview

Type	Full Name	Coverage	Ownership	Notes
LAN	Local Area Network	Single building or campus	Private	High speed (100 Mbps – 10 Gbps); owned and managed by the organisation
WAN	Wide Area Network	City, country, or global	Public/leased (ISPs, telecoms)	Variable speed; links multiple LANs across large distances
WLAN	Wireless LAN	Same as LAN but wireless	Private	Uses Wi-Fi (IEEE 802.11); no physical cables for end devices
PAN	Personal Area Network	A few metres around a person	Private	Connects personal devices (phone, laptop, smartwatch) via Bluetooth or USB
SAN	Storage Area Network	Data centre / enterprise	Private	High-speed network dedicated to connecting servers to shared storage devices; appears as local storage to servers
VPN	Virtual Private Network	Any geographic scale	Private (over public internet)	Creates an encrypted tunnel over the public internet, allowing remote users to access a private network securely as if locally connected

The internet is the largest WAN — a global network of interconnected networks using standardised protocols.

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Network Topologies

A network topology describes how devices (nodes) are physically or logically connected. The four main topologies in IB CS are bus, star, ring, and mesh.

Bus Topology

All devices connect to a single shared cable (the bus). Data travels in both directions along the bus; terminators at each end absorb signals to prevent reflection.

  [A]---[B]---[C]---[D]---[E]
  |                         |
terminator               terminator

Pros	Cons
Simple and cheap to install	A break anywhere in the cable takes down the whole network
Requires less cable than star	Performance degrades as more devices are added (collisions)
Easy to extend	Difficult to troubleshoot

Star Topology

All devices connect to a central switch or hub. Data passes through the central device to reach its destination.

      [A]
       |
  [B]--[Switch]--[C]
       |
      [D]

Pros	Cons
A cable failure only affects one device	If the central switch fails, the entire network goes down
Easy to add or remove devices	Requires more cable than bus
Easier to diagnose faults	Switch is a single point of failure
Better performance under heavy traffic (dedicated connections via switch)

Ring Topology

Devices are connected in a closed loop. Data travels in one direction (or both in dual-ring) around the ring; each device acts as a repeater.

  [A]--[B]
  |      |
  [D]--[C]

Pros	Cons
No data collisions (token passing controls access)	A break in the ring can take down the network (unless dual-ring)
Performance is predictable	Adding or removing devices disrupts the network
Signals can be regenerated at each node	More complex to manage than bus

Mesh Topology

Every device is connected directly to every other device (full mesh) or to multiple other devices (partial mesh). Data can take many routes between any two nodes.

  [A]---[B]
  |\ /\ |
  | X  X|
  |/ \/ |
  [C]---[D]

Pros	Cons
Highly fault-tolerant: multiple paths mean no single point of failure	Expensive — many cables and network ports required
Data can be rerouted if a link fails	Complex to install and manage
High redundancy ensures reliability	Rarely used for full mesh except in critical infrastructure

Common uses: Wide area network backbones, military communications, internet core infrastructure (partial mesh), and wireless mesh networks (e.g., smart home mesh Wi-Fi systems).

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Network Hardware

Understanding the role of each hardware component is essential for Paper 1 questions.

Devices and Their Roles

Device	Role
Router	Connects different networks (e.g., LAN to the internet); forwards packets between networks using IP addresses; assigns local IP addresses via DHCP
Switch	Connects devices within a LAN; sends data only to the specific destination device using MAC addresses (unlike a hub)
Hub	Connects devices in a LAN; broadcasts all data to every connected device regardless of destination (inefficient; largely obsolete)
Access Point (AP)	Extends a wired LAN wirelessly; devices connect via Wi-Fi to the AP, which connects to the network via Ethernet
NIC	Network Interface Card — hardware inside each device that enables it to connect to a network; has a unique MAC address burned in at manufacture
Modem	Modulates/demodulates signals to convert digital data to/from analogue signals for transmission over telephone or cable lines; used to connect to an ISP

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Protocols

A protocol is a set of agreed rules that govern how data is transmitted between devices on a network. Without common protocols, devices from different manufacturers could not communicate.

Key Protocols Table

Protocol	Full Name	Purpose
HTTP	HyperText Transfer Protocol	Transfers web pages between server and browser (unencrypted)
HTTPS	HTTP Secure	Same as HTTP but with TLS/SSL encryption — data cannot be read by third parties
FTP	File Transfer Protocol	Transfers files between a client and a server; supports upload and download; unencrypted by default (SFTP adds encryption)
SMTP	Simple Mail Transfer Protocol	Sends email from a client to a mail server, and between mail servers; used for outgoing email only
TCP	Transmission Control Protocol	Reliable, connection-oriented transport; guarantees delivery, ordering, and error checking via acknowledgements
IP	Internet Protocol	Addressing and routing packets across networks using IP addresses
DNS	Domain Name System	Translates human-readable domain names (e.g., studyforge.com) into IP addresses
DHCP	Dynamic Host Configuration Protocol	Automatically assigns IP addresses, subnet masks, and gateway addresses to devices joining a network

TCP/IP Four-Layer Model

The TCP/IP model (also called the Internet model) describes how data is processed as it moves from application to physical network and back.

Layer	Name	Responsibility	Example Protocols
4	Application	Provides network services to end-user applications	HTTP, HTTPS, DNS, DHCP, FTP, SMTP
3	Transport	End-to-end communication; segmentation, reliability, flow control	TCP, UDP
2	Internet	Logical addressing and routing of packets between networks	IP
1	Network Access (Link)	Physical transmission of data over the local network medium	Ethernet, Wi-Fi (IEEE 802.11)

When data is sent, each layer encapsulates the data from the layer above by adding its own header. On the receiving side, each layer decapsulates (removes the header) and passes the data up.

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IP Addressing

Every device on a network requires a unique IP address to send and receive data. IP addressing provides the logical addressing that enables routing across networks.

IPv4

IPv4 addresses are 32-bit values written as four decimal octets separated by dots, for example: 192.168.1.105

Each octet represents 8 bits, with a value from 0 to 255.

• Network portion — identifies the network (determined by the subnet mask)
• Host portion — identifies the specific device on that network

Total possible IPv4 addresses: $2^{32} \approx 4.3$ billion — this is now insufficient for the global internet, which is why IPv6 was developed.

Subnet Basics

A subnet mask (e.g., 255.255.255.0) indicates which bits of the IP address identify the network and which identify the host. Devices on the same subnet can communicate directly; devices on different subnets communicate via a router.

IPv6

IPv6 addresses are 128-bit values written in eight groups of four hexadecimal digits, for example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334

IPv6 provides $2^{128}$ possible addresses — effectively unlimited for the foreseeable future. IPv6 also includes built-in security features and simplified routing.

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Client-Server vs Peer-to-Peer

Networks can be organised around two fundamental architectures.

Client-Server

A server provides services or resources; clients request and consume them.

• Server is a dedicated, always-on machine with high performance
• Centralised management: data, security, and backups controlled from one point
• Scales well: many clients can share server resources
• Single point of failure: if the server goes down, clients lose access

Examples: web servers (HTTP), email servers (SMTP), file servers, authentication servers (Active Directory)

Peer-to-Peer (P2P)

All devices are equal (peers) and can act as both client and server simultaneously.

• No dedicated server: each device shares its own resources directly
• Decentralised: no single point of failure
• Cheap to set up: no server hardware required
• Harder to manage: security, backups, and permissions must be configured on each device
• Performance degrades as load increases on individual devices

Examples: BitTorrent file sharing, some online gaming networks, older home networks

Attribute	Client-Server	Peer-to-Peer
Management	Centralised	Distributed (each peer)
Cost	High (server hardware)	Low
Security	Easier to control	Harder to enforce uniformly
Reliability	Depends on server uptime	No single point of failure
Scalability	High	Limited by individual peer capacity

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Data Transmission and Packet Switching

Rather than sending data as a continuous stream, the internet breaks data into small units called packets.

Packet Switching

In packet switching, each packet is routed independently across the network and may take different paths to reach the destination. Packets are reassembled in the correct order at the destination.

Advantages of packet switching:

• Network resources are used efficiently (no dedicated line needed for each conversation)
• If one path fails, packets are rerouted automatically
• Multiple conversations can share the same links simultaneously

Packet Structure

Each packet contains three sections:

Section	Contents
Header	Source IP address, destination IP address, sequence number, protocol, TTL (time to live)
Payload	The actual data being transmitted (a chunk of the file, web page, etc.)
Trailer	Error-checking information (checksum); some protocols omit the trailer

Bandwidth vs Latency

Term	Definition	Analogy
Bandwidth	The maximum amount of data that can be transmitted per second (Mbps or Gbps)	Width of a pipe
Latency	The time delay for a packet to travel from source to destination (milliseconds)	Length of the pipe

High bandwidth but high latency = large files transfer quickly overall, but each request takes time to begin. Low latency is critical for real-time applications (video calls, online gaming).

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Network Security

Protecting networks from threats is a core syllabus area. Students must know both the types of threats and the corresponding protective measures.

Common Threats

Threat	Description
Malware	Malicious software including viruses (self-replicating, attach to files), worms (self-replicating, spread via network), ransomware (encrypts user data, demands payment), trojans (disguised as legitimate software)
Phishing	Deceptive emails or websites that trick users into revealing passwords or financial information
Denial of Service (DoS)	Flooding a server with traffic to make it unavailable to legitimate users; DDoS uses many compromised machines simultaneously
Man-in-the-Middle (MitM)	An attacker intercepts and potentially alters communication between two parties without their knowledge
SQL Injection	Malicious SQL code inserted into input fields to manipulate a database
Social Engineering	Manipulating people (rather than systems) into revealing confidential information

Protective Measures

Protection	How It Helps
Firewall	Monitors and filters incoming/outgoing network traffic based on rules; blocks unauthorised access
Encryption	Transforms data into an unreadable ciphertext; only parties with the correct key can decrypt it — protects data in transit and at rest
HTTPS / TLS	Encrypts all data between the browser and web server using TLS; prevents MitM interception of web traffic
VPN	Virtual Private Network — creates an encrypted tunnel between the user and a remote server, hiding traffic from ISPs and local eavesdroppers
Two-Factor Authentication (2FA)	Requires a second verification step (e.g., code sent to phone) in addition to a password — protects against stolen passwords
Antivirus / Anti-malware	Detects and removes known malware signatures; monitors for suspicious behaviour
Regular software updates	Patches known security vulnerabilities that attackers could exploit

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The Web

The World Wide Web (web) is an application that runs on top of the internet — it is a system of interlinked documents and resources accessed via web browsers using HTTP/HTTPS. The internet is the underlying network infrastructure; the web is one service that uses it.

Client-Server Model on the Web

The web uses a client-server model:

1. The user types a URL into a browser (client)
2. The browser resolves the domain name via DNS to get the server’s IP address
3. The browser sends an HTTP/HTTPS request to the web server
4. The web server processes the request and returns the requested resource (HTML, image, data)
5. The browser renders the received HTML, CSS, and JavaScript to display the page

URLs

A URL (Uniform Resource Locator) is the address of a resource on the web. Structure:

https://www.example.com:443/path/page.html?query=1#section
 |          |             |       |             |      |
scheme    domain         port    path          query  fragment

• Scheme — http or https (indicates the protocol)
• Domain — human-readable server address (resolved by DNS)
• Port — optional; defaults to 80 for HTTP, 443 for HTTPS
• Path — location of the specific resource on the server
• Query string — parameters passed to the server (key=value pairs)
• Fragment — references a section within the page (processed by the browser, not sent to server)

Web Technologies: HTML, CSS, and JavaScript

Modern web pages are built using three complementary technologies:

Technology	Role	Location
HTML (HyperText Markup Language)	Defines the structure and content of a page — headings, paragraphs, links, images, tables	Parsed by browser
CSS (Cascading Style Sheets)	Controls the visual presentation — colours, fonts, layout, spacing	Applied by browser
JavaScript	Adds interactivity and dynamic behaviour — responding to user actions, updating content without reloading the page	Executed by browser’s JS engine

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HL The OSI Model

The OSI (Open Systems Interconnection) model is a conceptual framework that divides network communication into seven distinct layers. Unlike the TCP/IP model (4 layers), the OSI model provides a more granular theoretical breakdown used in analysis and troubleshooting.

OSI Seven Layers

Layer (Number)	Responsibility	Protocols / Examples
Application (7)	Provides network services directly to user applications	HTTP, HTTPS, FTP, SMTP, DNS
Presentation (6)	Data translation, encryption/decryption, compression	SSL/TLS, JPEG, ASCII
Session (5)	Establishes, manages, and terminates communication sessions	NetBIOS, RPC
Transport (4)	End-to-end reliability, segmentation, flow control, error recovery	TCP, UDP
Network (3)	Logical addressing, routing packets between networks	IP
Data Link (2)	Physical addressing (MAC), error detection within a LAN frame	Ethernet, Wi-Fi (IEEE 802.11)
Physical (1)	Transmission of raw bits over a physical medium	Cables, hubs, radio signals

OSI vs TCP/IP Mapping

OSI Layers	TCP/IP Layer
Application + Presentation + Session	Application
Transport	Transport
Network	Internet
Data Link + Physical	Network Access (Link)

Subnetting Basics

A subnet (subnetwork) is a logical subdivision of an IP network. Subnetting allows a network administrator to divide a large network into smaller, more manageable segments.

The subnet mask works alongside the IP address to identify the boundary between the network portion and the host portion of an address. Written in the same dotted decimal format as an IP address (e.g., 255.255.255.0) or in CIDR notation (e.g., /24).

Example: IP address 192.168.1.45 with subnet mask 255.255.255.0 (or /24)

• Network portion: 192.168.1 (first 24 bits)
• Host portion: .45 (last 8 bits)
• Valid hosts on this subnet: 192.168.1.1 to 192.168.1.254
• Network address: 192.168.1.0; Broadcast address: 192.168.1.255

Why subnet? Subnetting reduces broadcast traffic (broadcasts stay within the subnet), improves security (each subnet can have its own firewall rules), and enables more efficient use of IP address space.

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Practice Questions

Q1 — Describe the role of a router on a home network. [3 marks]

Model answer:

A router connects the home LAN to the internet (WAN) (1 mark). It forwards data packets between these two networks using IP addresses to determine the correct destination (1 mark). In most home setups it also runs DHCP, automatically assigning local IP addresses to devices joining the network (1 mark).

Q2 — Explain one advantage and one disadvantage of a star topology compared to a bus topology. [4 marks]

Model answer:

Advantage: In a star topology, a cable fault only disconnects the single device attached to that cable (1 mark), whereas in a bus topology a break in the main cable disrupts all devices on the network (1 mark).

Disadvantage: The star topology has a single point of failure at the central switch — if the switch fails, all devices lose connectivity (1 mark), whereas in a bus topology there is no central device whose failure alone takes down the network (1 mark).

Q3 — State the purpose of DNS and explain what happens when a user types a web address into their browser. [4 marks]

Model answer:

DNS (Domain Name System) translates human-readable domain names into IP addresses (1 mark). When a user types www.example.com, the browser sends a query to a DNS server (1 mark). The DNS server looks up the domain and returns the corresponding IP address (e.g., 93.184.216.34) (1 mark). The browser then uses this IP address to establish a connection to the web server (1 mark).

Q4 — A student uses public Wi-Fi in a café to access their online banking. Identify two security risks and suggest one protective measure for each. [4 marks]

Model answer:

Risk 1: Man-in-the-middle attack — an attacker on the same Wi-Fi network could intercept the unencrypted traffic between the student and the bank. Protection: use HTTPS connections (padlock icon in browser), which encrypts data with TLS so an interceptor cannot read it (1 mark each).

Risk 2: Eavesdropping on unencrypted traffic — any device on a public network can potentially see packets broadcast over the access point. Protection: use a VPN, which creates an encrypted tunnel so all traffic is unreadable to others on the local network (1 mark each).

Q5 — Explain the difference between bandwidth and latency. Give one situation where each matters more than the other. [4 marks]

Model answer:

Bandwidth is the maximum amount of data that can be transmitted per second (1 mark). It matters more when downloading large files — a higher bandwidth connection completes large transfers faster (1 mark).

Latency is the time delay for a data packet to travel from source to destination (1 mark). It matters more in real-time applications such as video calling or online gaming, where even a short delay causes noticeable lag regardless of how much bandwidth is available (1 mark).

Q6 — State the difference between a DoS attack and a DDoS attack and explain why DDoS is harder to defend against. [3 marks]

Model answer:

A DoS (Denial of Service) attack floods a target server with traffic from a single source, making it unavailable to legitimate users (1 mark). A DDoS (Distributed DoS) attack sends the same flood from many compromised devices simultaneously (a botnet) (1 mark). DDoS is harder to defend against because the traffic originates from thousands of different IP addresses worldwide, making it extremely difficult to block all sources without also blocking legitimate traffic (1 mark).