Networking: Internet, OSI & Switching
This lesson covers three key networking areas: the difference between the Internet, WWW, and Intranet; the OSI 7-layer model that standardizes network communication; and the two methods of data transmission — circuit switching and packet switching.
Learning Objectives
- 12.3.6.1 Distinguish between the Internet, WWW, and Intranet
- 12.3.6.2 Explain the purpose and layers of the OSI model
- 12.3.6.3 Compare circuit switching and packet switching
Part 1: Internet, WWW & Intranet
| Concept | Definition | Key Feature |
|---|---|---|
| Internet | A global network of interconnected networks using TCP/IP | The physical infrastructure — cables, routers, servers |
| World Wide Web (WWW) | A collection of interconnected web pages accessed via the Internet | A service that runs ON the Internet (uses HTTP/HTTPS) |
| Intranet | A private network within an organisation, using Internet technologies | Restricted access — only for employees/members |
Road Analogy
The Internet is the road network. The WWW is the delivery service that uses those roads (like Amazon deliveries). An Intranet is a private gated community with its own internal roads — it uses the same road-building technology but is restricted to residents.
| Feature | Internet | WWW | Intranet |
|---|---|---|---|
| Scope | Global | Global (public) | Organisation only |
| Access | Public | Public | Private (login required) |
| Content | All network services | Web pages (HTML/CSS/JS) | Internal documents, tools |
| Protocols | TCP/IP | HTTP/HTTPS, HTML | Same as Internet/WWW |
| Example | The global network | google.com, wikipedia.org | Company portal, school LMS |
Common Exam Mistake
"The Internet and the WWW are the same thing" — FALSE. The Internet is the hardware/infrastructure. The WWW is a service (collection of web pages) that runs on the Internet. Email, FTP, gaming — these also use the Internet but are NOT part of the WWW.
Part 2: The OSI Model
The Open Systems Interconnection (OSI) model is a conceptual framework with 7 layers that standardizes how different network systems communicate. Each layer has a specific role and communicates with the layers directly above and below it.
| # | Layer | Function | Example Protocol/Device |
|---|---|---|---|
| 7 | Application | User interface — provides network services to applications | HTTP, FTP, SMTP, DNS |
| 6 | Presentation | Data formatting — encryption, compression, translation | SSL/TLS, JPEG, ASCII |
| 5 | Session | Manages sessions — establishes, maintains, and terminates connections | NetBIOS, RPC |
| 4 | Transport | End-to-end delivery — segmentation, error checking, flow control | TCP, UDP |
| 3 | Network | Routing — determines path from source to destination (logical addressing) | IP, ICMP, Routers |
| 2 | Data Link | Node-to-node transfer — frames, MAC addressing, error detection | Ethernet, Wi-Fi, Switches |
| 1 | Physical | Physical transmission — raw bits over cable/wireless | Cables, hubs, signals |
Mnemonic (Top → Bottom)
All People Seem To Need Data Processing — Application, Presentation, Session, Transport, Network, Data Link, Physical
Why Use the OSI Model?
- Standardisation — different manufacturers can build compatible equipment
- Modular design — changes in one layer don't affect others
- Troubleshooting — isolate problems to a specific layer
- Interoperability — devices from different vendors can communicate
✓ Data Flow: Sending an Email
Sender (Top → Bottom):
Layer 7: User composes email in Outlook (Application)
Layer 6: Email text is encoded/encrypted (Presentation)
Layer 5: Session established with mail server (Session)
Layer 4: Data split into segments, port number added (Transport)
Layer 3: IP addresses added, route determined (Network)
Layer 2: MAC addresses added, framed for local network (Data Link)
Layer 1: Electrical/optical signals sent over cable (Physical)
↓↓↓ Transmitted over network ↓↓↓
Receiver (Bottom → Top):
Layer 1 → 2 → 3 → 4 → 5 → 6 → 7
(Each layer strips its header and passes data up)Part 3: Circuit Switching vs Packet Switching
Circuit Switching
A dedicated communication path is established between sender and receiver for the entire duration of the communication. The path remains reserved even during silence.
| ✅ Advantages | ❌ Disadvantages |
|---|---|
| Guaranteed bandwidth — no congestion | Wastes bandwidth during silence |
| Constant data rate — predictable performance | Connection setup takes time |
| No packet reordering needed | If the path breaks, entire connection fails |
| Good for real-time communication (voice calls) | Expensive — dedicated resources per connection |
Example: Traditional telephone system (PSTN) — a circuit is established for the entire phone call.
Packet Switching
Data is broken into packets, each with a header containing the destination address. Packets can take different routes and are reassembled at the destination.
| ✅ Advantages | ❌ Disadvantages |
|---|---|
| Efficient bandwidth use — shared network | Packets may arrive out of order (need reassembly) |
| Robust — if one route fails, packets reroute | Variable delay (latency) — not ideal for real-time |
| No connection setup needed | Overhead — each packet carries header information |
| Multiple users can share the same network | Possible packet loss — requires error checking |
Example: The Internet — all web traffic, email, and streaming use packet switching.
✓ Packet Structure
┌──────────────────────────────────────────────────┐
│ PACKET │
├──────────────┬──────────────────┬────────────────┤
│ HEADER │ PAYLOAD │ TRAILER │
│ │ │ │
│ • Source IP │ • Actual data │ • Error check │
│ • Dest IP │ (part of the │ (checksum) │
│ • Packet # │ message) │ │
│ • TTL │ │ │
│ • Protocol │ │ │
└──────────────┴──────────────────┴────────────────┘Head-to-Head Comparison
| Feature | Circuit Switching | Packet Switching |
|---|---|---|
| Path | Dedicated, fixed path | Multiple routes, dynamic |
| Setup | Required before data transfer | No setup needed |
| Bandwidth | Reserved (may be wasted) | Shared (used efficiently) |
| Data order | Data arrives in order | Packets may arrive out of order |
| Failure | Entire connection fails | Packets reroute around failure |
| Delay | Consistent (low jitter) | Variable (may have jitter) |
| Cost | Expensive | Cost-effective |
| Best for | Voice calls, video calls | Web browsing, email, file transfer |
| Example | Phone network (PSTN) | Internet (TCP/IP) |
Pitfalls & Common Errors
Saying Internet = WWW
The Internet is the infrastructure (hardware, cables, protocols). The WWW is a service on the Internet. Email uses the Internet but NOT the WWW.
Mixing Up OSI Layer Functions
Learn each layer's one-sentence purpose. Common mix-up: Network layer (IP, routing) vs Data Link layer (MAC, framing). Network = logical addressing. Data Link = physical addressing.
Forgetting Packet Header Contents
If asked what a packet contains — always mention: source IP, destination IP, packet number (sequence), TTL, protocol, and the actual data (payload).
Pro-Tips for Exams
Networking Strategy
- For Internet vs WWW: always say "Internet = infrastructure, WWW = service that uses it"
- For OSI: learn the mnemonic and the one-line function of each layer
- For switching comparisons: use the table format with at least 5 points
- Always give examples: "circuit switching is used in traditional phone calls"
- If asked about packet switching — mention: packets, different routes, reassembly, header information
Graded Tasks
Define: Internet, WWW, Intranet. List the 7 OSI layers in order.
Explain why the Internet and WWW are not the same. Give 2 services that use the Internet but not the WWW.
Describe the function of each OSI layer. For each, give one example protocol or device.
Compare circuit switching and packet switching across 5 features. Explain which is better suited for a video conference and why.
A company wants to set up internal communication. Should they use an intranet or just the public Internet? Discuss the advantages and security implications of each approach.