AQA 7517 · A-Level Computer Science · ~14 min read
The Internet vs The World Wide Web
These are often confused but are distinct:
The Internet: a global network of interconnected networks using IP (Internet Protocol). It is the infrastructure — cables, routers, satellites.
The World Wide Web (WWW): a service that runs on the internet. A collection of web pages accessed via HTTP/HTTPS using URLs.
Other services also run on the internet: email (SMTP/IMAP/POP3), FTP, VoIP, online gaming.
URLs and How They Work
A URL (Uniform Resource Locator) has components:
https://www.cszone.co.uk/lessons/4-9-4c/
Part
Example
Meaning
Protocol
https://
Secure HTTP — encrypted with TLS
Domain name
www.cszone.co.uk
Human-readable address, resolved by DNS
Top-level domain
.co.uk
Country-code TLD
Path
/lessons/4-9-4c/
Location of the resource on the server
Routing and Routing Tables
The internet routes packets from source to destination across many interconnected networks. Routers make forwarding decisions using routing tables.
A routing table maps destination IP ranges to next-hop addresses (which router to send the packet to next)
Routers use routing protocols (OSPF, BGP) to discover and update their routing tables
Default gateway: the router a device sends packets to when it doesn't have a specific route (typically your home router → ISP)
Packets from same message may take different routes across the internet — reassembled at destination
TTL (Time To Live): decremented at each router hop — packet discarded when TTL=0 (prevents infinite looping)
Cookies and Sessions
HTTP is stateless — each request is independent. Cookies and sessions maintain state:
Cookie: small text file stored by the browser. Set by the server via HTTP header. Sent back with every subsequent request to that domain. Stores: session IDs, preferences, tracking data.
Session: server-side data store linked to a session ID (in a cookie). Stores login state, shopping cart, etc. More secure — data stays on server; cookie only holds the ID.
First-party cookies: set by the domain you visit. Third-party cookies: set by other domains (advertising networks) — used for cross-site tracking.
Client sends "ClientHello" — lists supported cipher suites, TLS version
Server responds with its digital certificate (contains public key, signed by CA)
Client verifies certificate is from a trusted Certificate Authority
Client and server use asymmetric encryption to exchange a session key
All subsequent communication encrypted with the session key (symmetric encryption — fast)
Benefits: encryption prevents eavesdropping; authentication proves server identity; integrity via MAC detects tampering.
Internet Protocols Summary
Protocol
Port
Purpose
HTTP
80
Web page transfer (unencrypted)
HTTPS
443
Encrypted web page transfer (TLS)
FTP
20/21
File transfer
SSH
22
Secure remote terminal access
SMTP
25/587
Sending email
POP3
110
Downloading email (removes from server)
IMAP
143
Email access (keeps on server, syncs)
DNS
53
Domain name resolution
Exam tip: A common AQA 7517 exam question asks you to distinguish the internet from the WWW. Key: internet = infrastructure; WWW = service. Know what a routing table does — maps destination ranges to next-hop routers. Know the difference between IMAP (email stays on server, syncs across devices) and POP3 (downloads and deletes from server). Cookie vs session: cookie = browser-stored, session = server-stored. HTTPS TLS handshake: asymmetric to exchange symmetric session key.
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Worksheet — 4.9.4c The Internet
8 questions · instantly marked · AQA 7517 standard
Q1Explain the difference between the internet and the World Wide Web.[2]
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Internet: global network of interconnected networks — the physical infrastructure (cables, routers, ISPs) [1]; uses IP (Internet Protocol) to route packets [1]. World Wide Web: a service that runs on the internet [1]; a collection of web pages accessed via HTTP/HTTPS using URLs and browsers [1].
Q2Describe how a router uses a routing table to forward a packet across the internet.[2]
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Router receives a packet and reads the destination IP address in the header [1]; looks up the destination in its routing table to find the best next-hop router [1]; forwards the packet to that next hop [1]; routing tables are built/updated using routing protocols (OSPF, BGP) [1].
Q3Explain the purpose of the TTL field in an IP packet header.[2]
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TTL (Time To Live): a counter set when the packet is created [1]; decremented by 1 at each router hop [1]; when TTL reaches 0, the packet is discarded and an error message sent to sender — prevents packets circulating indefinitely on the internet [1].
Q4Explain the difference between a cookie and a session in the context of web applications.[3]
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Cookie: small text file stored by the browser on the user's device [1]; sent back to the server with each request to that domain — stores session IDs, preferences [1]. Session: server-side data store linked to a session ID [1]; more secure — actual user data stays on the server, cookie only holds the ID [1].
Q5Describe the TLS handshake process when a user connects to an HTTPS website.[4]
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1) Client sends ClientHello — lists supported cipher suites and TLS version [1]; 2) Server responds with its digital certificate containing its public key, signed by a Certificate Authority [1]; 3) Client verifies the certificate is from a trusted CA [1]; 4) Asymmetric encryption used to securely exchange/agree a symmetric session key [1]; 5) All subsequent communication encrypted with the session key (symmetric — fast) [1].
Q6A user checks their email on multiple devices. Explain why IMAP would be more appropriate than POP3.[2]
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IMAP: email remains on the server and is synchronised across all devices [1]; changes made on one device (read, delete, move) are reflected on all others [1]. POP3: downloads email to one device and typically deletes it from the server — not accessible on other devices [1].
Q7Identify four components of the URL https://www.cszone.co.uk/lessons/ and explain each.[3]
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Protocol — https:// — encrypted HTTP using TLS [1]; Domain name — www.cszone.co.uk — human-readable address resolved by DNS to an IP address [1]; Top-level domain — .co.uk — country-code TLD (UK commercial) [1]; Path — /lessons/ — location of the resource on the web server [1].
Q8Explain why HTTP is stateless and describe one mechanism used to work around this limitation.[2]
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Stateless: each HTTP request is independent — the server has no memory of previous requests from the same client [1]; this means without additional mechanisms, the server cannot tell if two requests are from the same user [1]. Mechanism — Cookies: the server sets a cookie on the client with a session ID [1]; this is sent back with each subsequent request, allowing the server to identify the user and maintain state (e.g. shopping cart, login) [1].
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Mini Test — The Internet
10 questions · 10 minutes
⏱ 10:00
Section A — Multiple Choice [5 marks]
Q1The World Wide Web is:
Q2The TTL field in an IP packet header:
Q3IMAP is preferred over POP3 when:
Q4A cookie is stored:
Q5HTTPS differs from HTTP because it:
Section B — Short Answer [5 marks]
Q6State two other services that use the internet besides the World Wide Web.
Mark schemeEmail (SMTP/IMAP/POP3) [1]; FTP (file transfer) [1]; VoIP (voice calls) [1]; online gaming [1]. Award any 2.
Q7What does a router use to decide where to forward a packet?
Mark schemeA routing table [1]; maps destination IP address ranges to next-hop router addresses [1].
Q8Why is HTTP described as stateless?
Mark schemeEach HTTP request is independent — the server has no memory of previous requests from the same client [1]; it cannot inherently identify if two requests come from the same user [1].
Q9State the port number used by HTTPS.
Mark schemePort 443 [1].
Q10In the TLS handshake, what type of encryption is used to protect the bulk of the session data (after key exchange)?
Mark schemeSymmetric encryption [1]; a symmetric session key is exchanged using asymmetric encryption, then used for the rest of the session because symmetric encryption is faster [1].