1.3.3 Networks
1.3.3(a) Network Characteristics
Networks allow computers to communicate and share resources. Understanding network types and protocols is fundamental to modern computing.
Types of Networks
Networks are classified by their geographical scope and the area they cover.
LAN (Local Area Network)
Definition
A LAN (Local Area Network) covers a small geographical area, typically a single building or campus. It is owned, managed, and maintained by a single organisation.
Key Point
| LAN characteristic | Details |
|---|---|
| Coverage | Small area (building/campus) |
| Typical speed | High speed (about 100Mbps to 10Gbps) |
| Ownership | Usually one organisation |
| Latency | Low latency / fast response |
| Typical hardware | Switches, routers, access points |
| Common examples | School, office, home network |
WAN (Wide Area Network)
Definition
A WAN (Wide Area Network) spans large geographical areas - cities, countries, or even continents. The Internet is the largest WAN.
Key Point
| WAN characteristic | Details |
|---|---|
| Coverage | Large area (city/country/global) |
| Infrastructure | Often leased telecom links |
| Ownership | May involve multiple organisations |
| Latency | Higher than LAN |
| Typical speed | Usually lower than LAN over distance |
| Common examples | Internet, multi-office enterprise WAN |
Other Network Types
Key Point
| Network type | Scope | Typical technology/example |
|---|---|---|
| PAN | Around one person | Bluetooth/NFC (phone ↔ earbuds) |
| MAN | City or large campus | City-wide Wi-Fi / municipal network |
Network Protocols
For devices to communicate, they must follow agreed rules - these are called protocols.
Definition
A protocol is a set of rules that governs how data is transmitted and received over a network. Protocols ensure devices from different manufacturers can communicate.
Key Point
| Protocol | Full name | Main purpose |
|---|---|---|
| HTTP | Hypertext Transfer Protocol | Access web pages (unencrypted) |
| HTTPS | HTTP Secure | Access web pages securely (encrypted) |
| TCP | Transmission Control Protocol | Reliable data delivery |
| IP | Internet Protocol | Addressing and routing |
| FTP | File Transfer Protocol | File transfer |
| SMTP | Simple Mail Transfer Protocol | Send email |
| POP3 | Post Office Protocol v3 | Download email (often removes from server) |
| IMAP | Internet Message Access Protocol | Manage email while keeping mail on server |
| DNS | Domain Name System | Domain name to IP translation |
1.3.3(b) Internet Structure
The Internet is a global network of networks, and understanding its structure is essential for computer science.
The TCP/IP Stack
Data transmission uses a layered model where each layer has specific responsibilities.
Definition
The TCP/IP stack (also called the Internet Protocol Suite) is a set of protocols organised into four layers that work together to enable network communication.
Key Point
| Layer (top→bottom) | Name | Main function | Example protocols |
|---|---|---|---|
| 4 | Application | User-facing services and data formatting | HTTP, FTP, SMTP, DNS |
| 3 | Transport | End-to-end delivery and reliability | TCP, UDP |
| 2 | Network/Internet | Addressing and routing between networks | IP, ICMP |
| 1 | Link/Network Interface | Local delivery and physical transmission | Ethernet, Wi-Fi |
Worked Example
Worked Example: Sending Data Through TCP/IP Layers
Scenario: You request a webpage (www.example.com)
Sending (going DOWN the stack):
Layer 4 - Application:
• Browser creates HTTP request for the webpage
• Data: "GET /index.html HTTP/1.1"
Layer 3 - Transport:
• TCP breaks data into segments
• Adds port numbers (source: 52481, destination: 80)
• Adds sequence numbers for reassembly
Layer 2 - Network:
• IP adds addresses (your IP → server's IP)
• Creates packets with routing information
Layer 1 - Link:
• Adds MAC addresses for local delivery
• Converts to electrical signals/radio waves
• Data transmitted across physical network
Receiving (going UP the stack):
The process reverses - each layer removes its header and passes data up until the browser displays the webpage.
Scenario: You request a webpage (www.example.com)
Sending (going DOWN the stack):
Layer 4 - Application:
• Browser creates HTTP request for the webpage
• Data: "GET /index.html HTTP/1.1"
Layer 3 - Transport:
• TCP breaks data into segments
• Adds port numbers (source: 52481, destination: 80)
• Adds sequence numbers for reassembly
Layer 2 - Network:
• IP adds addresses (your IP → server's IP)
• Creates packets with routing information
Layer 1 - Link:
• Adds MAC addresses for local delivery
• Converts to electrical signals/radio waves
• Data transmitted across physical network
Receiving (going UP the stack):
The process reverses - each layer removes its header and passes data up until the browser displays the webpage.
DNS (Domain Name System)
Definition
DNS (Domain Name System) translates human-readable domain names (like www.google.com) into IP addresses (like 142.250.187.46) that computers use to identify each other.
Worked Example
Worked Example: DNS Lookup
You type: www.example.com
Step 1: Browser checks local cache
• Have we looked this up recently? If yes, use cached IP.
Step 2: Query local DNS resolver (usually your ISP)
• Resolver checks its cache
Step 3: Query root DNS server
• "Where can I find .com domains?"
• Returns address of .com TLD server
Step 4: Query TLD (Top-Level Domain) server
• "Where can I find example.com?"
• Returns address of example.com's authoritative server
Step 5: Query authoritative DNS server
• "What's the IP for www.example.com?"
• Returns: 93.184.216.34
Step 6: IP address returned to browser
• Browser connects to 93.184.216.34
• Webpage loads
You type: www.example.com
Step 1: Browser checks local cache
• Have we looked this up recently? If yes, use cached IP.
Step 2: Query local DNS resolver (usually your ISP)
• Resolver checks its cache
Step 3: Query root DNS server
• "Where can I find .com domains?"
• Returns address of .com TLD server
Step 4: Query TLD (Top-Level Domain) server
• "Where can I find example.com?"
• Returns address of example.com's authoritative server
Step 5: Query authoritative DNS server
• "What's the IP for www.example.com?"
• Returns: 93.184.216.34
Step 6: IP address returned to browser
• Browser connects to 93.184.216.34
• Webpage loads
Key Point
Why DNS Matters:
• Humans remember names, not numbers
• Websites can change IP addresses without users knowing
• Load balancing - one domain can point to multiple IPs
• Hierarchical structure provides scalability
• Distributed system - no single point of failure
• Humans remember names, not numbers
• Websites can change IP addresses without users knowing
• Load balancing - one domain can point to multiple IPs
• Hierarchical structure provides scalability
• Distributed system - no single point of failure
Packet Switching vs Circuit Switching
Definition
Packet switching breaks data into small packets that are sent independently across the network and reassembled at the destination. The Internet uses packet switching.
Definition
Circuit switching establishes a dedicated communication path between two parties for the duration of their connection. Traditional telephone networks used circuit switching.
Key Point
| Feature | Packet switching | Circuit switching |
|---|---|---|
| Connection | No dedicated path | Dedicated path reserved |
| Routing | Packets may take different routes | Same route for whole session |
| Efficiency | High (shared network) | Lower (reserved capacity) |
| Reliability | Can route around failures | Connection fails if path breaks |
| Delay | Variable with congestion | More consistent once circuit set |
| Typical use | Internet/data networks | Traditional phone networks |
1.3.3(c) Network Security
Networks face numerous threats. Understanding security measures is crucial.
Firewalls
Definition
A firewall monitors and controls incoming and outgoing network traffic based on predetermined security rules. It acts as a barrier between trusted and untrusted networks.
Key Point
| Firewall function | What it checks/does |
|---|---|
| Packet filtering | Filters by source/destination IP, port, protocol |
| Stateful inspection | Tracks active sessions and allows valid return traffic |
| Application-layer filtering | Inspects higher-level content/patterns |
| Drop vs reject | Drop = silent discard, Reject = explicit refusal response |
Example
Firewall Rules Example:
Rule 1: ALLOW incoming traffic on port 80 (HTTP)
Rule 2: ALLOW incoming traffic on port 443 (HTTPS)
Rule 3: BLOCK incoming traffic on port 23 (Telnet - insecure)
Rule 4: ALLOW outgoing traffic on all ports
Rule 5: BLOCK all other incoming traffic (default deny)
Rule 1: ALLOW incoming traffic on port 80 (HTTP)
Rule 2: ALLOW incoming traffic on port 443 (HTTPS)
Rule 3: BLOCK incoming traffic on port 23 (Telnet - insecure)
Rule 4: ALLOW outgoing traffic on all ports
Rule 5: BLOCK all other incoming traffic (default deny)
Proxy Servers
Definition
A proxy server acts as an intermediary between clients and the internet. Requests go to the proxy, which forwards them to the destination.
Key Point
Proxy Server Benefits:
• Anonymity - Hides client's real IP address
• Caching - Stores frequently accessed content (faster access)
• Filtering - Can block access to certain websites
• Logging - Records all web activity
• Security - Adds layer between internal network and internet
• Anonymity - Hides client's real IP address
• Caching - Stores frequently accessed content (faster access)
• Filtering - Can block access to certain websites
• Logging - Records all web activity
• Security - Adds layer between internal network and internet
Encryption
Definition
Encryption converts data into an unreadable format (ciphertext) that can only be read by someone with the correct decryption key.
Key Point
Network Encryption:
• HTTPS - Encrypts web traffic (look for padlock icon)
• VPN - Encrypts all traffic through a tunnel
• WPA2/WPA3 - Encrypts Wi-Fi traffic
• Without encryption, data can be intercepted and read
• HTTPS - Encrypts web traffic (look for padlock icon)
• VPN - Encrypts all traffic through a tunnel
• WPA2/WPA3 - Encrypts Wi-Fi traffic
• Without encryption, data can be intercepted and read
1.3.3(d) Network Hardware
Different hardware devices serve different purposes in a network.
Network Interface Card (NIC)
Definition
A NIC (Network Interface Card) connects a device to a network. Every NIC has a unique MAC address.
Switch
Definition
A switch connects devices within a LAN. It uses MAC addresses to forward data only to the intended recipient, making it more efficient than a hub.
Router
Definition
A router connects different networks together and routes data between them using IP addresses. Your home router connects your LAN to the internet (WAN).
Other Hardware
Key Point
| Device | Main function |
|---|---|
| WAP (Wireless Access Point) | Connects wireless clients to wired LAN |
| Gateway | Connects networks using different protocols |
| Bridge | Connects LAN segments at Layer 2 |
| Modem | Converts digital/analogue signals for transmission medium |
Key Point
MAC Address vs IP Address:
MAC Address:
• Physical/hardware address
• Unique to each NIC (burned in at manufacture)
• 48 bits, shown as hex (e.g., 00:1A:2B:3C:4D:5E)
• Works at Link layer
• Doesn't change (usually)
IP Address:
• Logical/software address
• Assigned by network (DHCP) or manually
• IPv4: 32 bits (e.g., 192.168.1.1)
• Works at Network layer
• Can change when you connect to different networks
MAC Address:
• Physical/hardware address
• Unique to each NIC (burned in at manufacture)
• 48 bits, shown as hex (e.g., 00:1A:2B:3C:4D:5E)
• Works at Link layer
• Doesn't change (usually)
IP Address:
• Logical/software address
• Assigned by network (DHCP) or manually
• IPv4: 32 bits (e.g., 192.168.1.1)
• Works at Network layer
• Can change when you connect to different networks
1.3.3(e) Client-Server vs Peer-to-Peer
Networks can be organised in different ways depending on their purpose.
Client-Server Model
Definition
In a client-server network, dedicated servers provide services and resources to client computers that request them.
Key Point
Client-Server Characteristics:
• Central server(s) provide services
• Clients request services from server
• Centralised management and security
• Server requires powerful hardware
• Examples: Email servers, web servers, file servers
• Central server(s) provide services
• Clients request services from server
• Centralised management and security
• Server requires powerful hardware
• Examples: Email servers, web servers, file servers
Pros & Cons
Pros & Cons (Client-Server):
| Advantages | Disadvantages |
|---|---|
| Centralised management and backup | Higher setup cost |
| Better security (controlled at server) | Server is single point of failure |
| Easier to scale | Requires IT expertise to maintain |
| Consistent performance |
Peer-to-Peer Model
Definition
In a peer-to-peer (P2P) network, each device (peer) can act as both client and server, sharing resources directly with other peers.
Key Point
Peer-to-Peer Characteristics:
• No central server
• All devices are equal (peers)
• Resources shared directly between devices
• Decentralised
• Examples: BitTorrent, some gaming networks
• No central server
• All devices are equal (peers)
• Resources shared directly between devices
• Decentralised
• Examples: BitTorrent, some gaming networks
Pros & Cons
Pros & Cons (Peer-to-Peer):
| Advantages | Disadvantages |
|---|---|
| Low cost (no server needed) | Difficult to manage/secure |
| Easy to set up | Performance depends on peers being online |
| No single point of failure | No centralised backup |
| Scales naturally as peers join | Security harder to enforce |
Exam Tip
When to use each:
• Client-Server: Businesses, schools - need security, management, reliability
• Peer-to-Peer: Home networks, file sharing - simple, low cost
• Client-Server: Businesses, schools - need security, management, reliability
• Peer-to-Peer: Home networks, file sharing - simple, low cost