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.
Protocol Layering
Definition
Protocol layering divides network communication into separate layers. Each layer has one well-defined job and exchanges data only with the layers directly above and below it.
Key Point
Why layering is used:
• The protocols are applied in order, one after the other
• Each layer is self-contained, so one layer can be changed or replaced without affecting the others
• Each layer hides its details from the layers around it, which is a form of abstraction
• A large task is broken down into smaller, manageable units
• Faults are easier to find, because a problem can be traced to a single layer
• Standards can be developed for each layer separately, so products from different manufacturers work together
• The protocols are applied in order, one after the other
• Each layer is self-contained, so one layer can be changed or replaced without affecting the others
• Each layer hides its details from the layers around it, which is a form of abstraction
• A large task is broken down into smaller, manageable units
• Faults are easier to find, because a problem can be traced to a single layer
• Standards can be developed for each layer separately, so products from different manufacturers work together
Example
Layering in practice:
A web browser works in the same way whether the device is connected by Wi-Fi or by a cable. How the data physically travels is the job of the link layer, so the application layer above it does not have to change.
A web browser works in the same way whether the device is connected by Wi-Fi or by a cable. How the data physically travels is the job of the link layer, so the application layer above it does not have to change.
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 |