1.3.2b The Internet (IP Addressing & Packet Switching)
OCR J277 · GCSE Computer Science · ~14 min read
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The Internet vs the World Wide Web
Students often confuse these — they are different things:
The Internet
The World Wide Web (WWW)
A global network of interconnected networks (hardware infrastructure)
A service that runs on top of the internet — a collection of web pages linked by hyperlinks
Involves routers, cables, servers, WAPs
Uses HTTP/HTTPS to transfer web pages
Existed before the WWW (ARPANET, 1969)
Invented by Tim Berners-Lee in 1989
Email, VoIP, FTP also run on it
Accessed via a web browser (Chrome, Firefox, etc.)
IP Addressing
Every device connected to the internet is assigned an IP address — a unique numerical label used to identify and locate devices. There are two versions:
Feature
IPv4
IPv6
Format
Four 8-bit numbers separated by dots (e.g. 192.168.1.1)
Eight groups of 4 hexadecimal digits (e.g. 2001:0db8:85a3::8a2e:0370:7334)
Bit length
32 bits
128 bits
Total addresses
~4.3 billion (2³²)
~340 undecillion (2¹²⁸)
Status
Running out — being replaced
Designed to solve IPv4 exhaustion
IPv4 addresses are running out because the internet now has billions of connected devices (smartphones, IoT, etc.). IPv6 provides a vastly larger address space to solve this.
Packet Switching
Packet switching is the method used to transmit data over the internet. Data is broken into small chunks called packets, each sent independently across the network, and reassembled at the destination.
Structure of a Packet
Each packet contains:
Header: source IP address, destination IP address, packet number, total number of packets, TTL (Time To Live)
Payload: the actual data (portion of the original file/message)
Trailer (optional): error checking (checksum)
How Packet Switching Works
Data is split into packets (typically up to 1,500 bytes each)
Each packet is addressed with source and destination IP addresses
Packets travel across the network, each potentially taking a different route
Routers use routing tables to direct each packet along the best available path
Packets may arrive out of order — TCP reassembles them in the correct sequence
If a packet is lost or corrupted, only that packet is resent (not the whole file)
📦 Worked Example
A 10 MB image file is broken into thousands of packets. Packet 1 might travel London → Paris → New York; Packet 2 might go London → Dublin → New York. Both arrive at the destination and TCP reassembles them in the correct order using the packet numbers in each header.
Advantages of Packet Switching
Resilient: if one route fails, packets can take an alternative path
Efficient: network bandwidth is shared — multiple communications can use the same links simultaneously
Only lost packets need retransmission, not the entire file
No dedicated circuit needed between sender and receiver
Routers and Routing
A router is the key device in packet switching. Routers:
Receive packets and read the destination IP address in the header
Consult their routing table to determine the best next hop
Forward the packet towards its destination
Connect different networks together (e.g. your LAN to the internet)
The routing table lists known network destinations and the next router to send packets to. Routers constantly update their tables to reflect current network conditions.
URL, Domain Names and Web Hosting
A URL (Uniform Resource Locator) is a complete web address, e.g. https://www.cszone.co.uk/lessons/1-3-2b
https:// — the protocol
www.cszone.co.uk — the domain name (resolved by DNS to an IP address)
/lessons/1-3-2b — the path to the specific resource on the server
Web pages are stored on web servers. When you type a URL, your browser sends an HTTP/HTTPS request to that server, which responds with the web page.
ISPs and the Internet Infrastructure
An ISP (Internet Service Provider) connects homes and businesses to the internet. ISPs connect to each other via high-speed backbone networks (primarily fibre optic cables). The internet backbone consists of extremely high-bandwidth links connecting major network hubs around the world.
Exam tip: Know the difference between the internet and the WWW. Packet switching is a key topic — know what a packet contains (header with source/destination IP, packet number; payload = data), why packets may take different routes, and how TCP reassembles them. IPv4 vs IPv6 — know IPv4 is 32-bit (exhausted), IPv6 is 128-bit (solution). Routing table questions come up — routers use them to forward packets.
⚠️ Common Mistakes
Confusing the internet with the WWW — the internet is the global network infrastructure; WWW is a service running on it
Saying all packets take the same route — they don't; each packet may travel independently via different paths
Forgetting that TCP reassembles packets in the correct order at the destination
Confusing IP (addressing/routing) with TCP (reliable delivery/reassembly) — they work together but have different jobs
Saying IPv4 has run out completely — it hasn't fully, but addresses are extremely scarce and IPv6 is the solution
✅ Notes completed!
▶
Video coming soon
What's in this video
• Internet vs WWW: what's different?
• IPv4 and IPv6: why we ran out of addresses
• Packet switching animation: how data travels in packets
• Routers and routing tables: finding the best path
Click slide or press arrow keys to navigate
Worksheet — 1.3.2b The Internet
8 questions · 22 marks
Q1Explain the difference between the internet and the World Wide Web.[2]
✅ Mark scheme
The internet is the global physical network infrastructure (hardware: cables, routers, servers) [1]; the World Wide Web is a service/collection of web pages that runs on top of the internet, accessed via a browser using HTTP/HTTPS [1].
Q3Why was IPv6 developed? What problem does it solve?[2]
✅ Mark scheme
IPv4 addresses are running out due to the vast number of devices connected to the internet [1]; IPv6 provides a much larger address space (128-bit = 2¹²⁸ addresses) to ensure there are enough unique addresses for all devices [1].
Q4Describe what packet switching is.[3]
✅ Mark scheme
Data is broken into small chunks called packets [1]; each packet contains a header (source/destination IP, packet number) and the data (payload) [1]; packets are sent independently across the network, possibly via different routes, and reassembled in the correct order at the destination [1].
Q5State three pieces of information found in a packet header.[3]
✅ Mark scheme
Any three from: source IP address [1]; destination IP address [1]; packet number [1]; total number of packets [1]; TTL (Time To Live) [1].
Q6Explain two advantages of packet switching over dedicated circuit switching.[4]
✅ Mark scheme
Resilient — if one route fails, packets can take an alternative path so communication continues [1+1]; Efficient — bandwidth is shared; multiple communications can use the same links simultaneously, unlike a dedicated circuit which reserves the line [1+1]; Only lost packets need retransmitting, not the whole file [1].
Q7Describe the role of a router in packet switching.[3]
✅ Mark scheme
Routers receive packets and read the destination IP address in the header [1]; they consult their routing table to determine the best next hop (route) to forward the packet [1]; they forward the packet toward the destination, potentially choosing different routes for different packets [1].
Q8Break down the URL https://www.ocr.org.uk/qualifications/gcse into its components, explaining what each part means.[3]
✅ Mark scheme
https:// — the protocol used to transfer the web page (secure HTTP) [1]; www.ocr.org.uk — the domain name, which DNS resolves to an IP address [1]; /qualifications/gcse — the path to the specific resource/page on the web server [1].
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out of 22 — self-mark above
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Mini Test — 1.3.2b The Internet
10 questions · 10 marks · 10 minutes
⏱ 10:00
10 marks
Section A — Multiple Choice [5 marks]
Q1How many bits does an IPv4 address use?
Q2What is packet switching?
Q3What does a packet header contain?
Q4Who invented the World Wide Web?
Q5Why was IPv6 developed?
Section B — Short Answer [5 marks]
Q6State one difference between the internet and the World Wide Web.
Mark schemeThe internet is the physical global network infrastructure; the WWW is a service (collection of web pages) running on the internet, accessed via a browser. [1]
Q7State two advantages of packet switching.
Mark schemeIf one route fails, packets can take an alternative path [½]; bandwidth is shared so multiple communications can use the same links [½]; only lost packets need retransmitting [½]. (Any two, ½ mark each.) [1]
Q8What is the role of a router in the internet?
Mark schemeA router reads the destination IP address in each packet's header, consults its routing table, and forwards the packet to the next hop towards its destination. [1]
Q9What does the 'payload' part of a packet contain?
Mark schemeThe payload contains the actual data being transmitted (a portion of the original file or message). [1]
Q10An IPv4 address is 32 bits. How many unique addresses does this allow? Give your answer as a power of 2.
Mark scheme2³² = approximately 4.3 billion unique addresses. [1]