Explain how a Uniform Resource Locator (URL) is used to locate a resource on the World Wide Web (WWW) and the role of the Domain Name Service (DNS)

2.1 Networks – The Internet

Objective

Explain how a Uniform Resource Locator (URL) is used to locate a resource on the World Wide Web (WWW) and describe the role of the Domain Name Service (DNS). In addition, understand the underlying networking concepts required by the Cambridge International AS & A Level Computer Science (9618) syllabus.

1. Internet Architecture – The TCP/IP Stack

2. What Is a URL?

A Uniform Resource Locator (URL) is a textual address that tells a web client (normally a browser) what resource is required and how to obtain it.

2.1 Components of a URL

3. How a URL Locates a Resource

1. Parsing – The browser separates the URL into its components.
2. Select protocol handler – The scheme determines which application‑layer protocol (HTTP, HTTPS, FTP, etc.) will be used.
3. Domain name resolution – The host part (www.example.com) is sent to the DNS resolver to obtain an IP address (IPv4 or IPv6).
4. Establish transport connection – Using the returned IP address and the required port, the client opens a TCP (or UDP) connection.
5. Application‑layer request – For HTTP/HTTPS the client sends a request line that includes the path and query string.
6. Server response – The server returns the requested resource (HTML, image, video, etc.) or an error status.
7. Fragment handling – If a fragment identifier is present, the browser scrolls to the corresponding element after rendering.

Sample Pseudo‑code for a Recursive DNS Resolver (AO2 style)

function resolve(hostname):
if cache.contains(hostname):
return cache.get(hostname)
// query the configured recursive resolver
answer = queryRecursiveResolver(hostname)
if answer.isAuthoritative():
cache.store(hostname, answer.address, answer.TTL)
return answer.address
else:
// fallback – perform an iterative lookup starting at the root
return iterativeLookup(hostname)

4. Example Walk‑through (IPv4 & IPv6)

Resolve https://www.example.com/articles/ai/introduction.html?lang=en#overview

1. Parse – scheme = https; host = www.example.com; path = /articles/ai/introduction.html; query = lang=en; fragment = overview.
2. DNS query – Resolver asks the recursive DNS server for an A (IPv4) and an AAAA (IPv6) record.
3. Resolution chain – Root → .com TLD → authoritative server for example.com → returns:

   • A: 93.184.216.34
   • AAAA: 2606:2800:220:1:248:1893:25c8:1946

4. Transport – Browser opens a TCP connection to the chosen address (e.g., IPv4 93.184.216.34) on port 443.
5. HTTPS handshake – TLS negotiation establishes an encrypted channel.
6. HTTP request:

   GET /articles/ai/introduction.html?lang=en HTTP/1.1
   Host: www.example.com
   Connection: close

7. Response – Server returns the HTML document; the browser renders it and jumps to the element with id="overview".

5. Domain Name Service (DNS)

• Distributed hierarchical database – Root → Top‑Level Domains (TLDs) → Second‑level domains → Sub‑domains.
• Resolver process – Local cache → configured recursive resolver → authoritative name servers.
• Common record types

  • A – IPv4 address.
  • AAAA – IPv6 address.
  • CNAME – Canonical name (alias).
  • MX – Mail exchange (used by SMTP).
  • NS – Name‑server delegation.
  • TXT – Arbitrary text (often used for SPF, DKIM).

• TTL (Time‑to‑Live) – Determines how long a record may be cached before a fresh query is required.
• DNS caching – Browsers, operating systems and recursive resolvers cache records to speed up look‑ups; stale entries can cause “wrong‑site” errors if TTLs are too long.
• DNSSEC – Adds digital signatures to DNS records, allowing resolvers to verify authenticity and protect against spoofing attacks.
• Security considerations

  • Cache poisoning – an attacker injects false records into a resolver’s cache.
  • Man‑in‑the‑middle attacks – mitigated by DNSSEC and TLS/HTTPS.

6. IP Addressing Fundamentals

6.1 IPv4 vs. IPv6

6.2 Public vs. Private IPv4 Ranges

• 10.0.0.0 / 8 – 10.0.0.0 – 10.255.255.255
• 172.16.0.0 / 12 – 172.16.0.0 – 172.31.255.255
• 192.168.0.0 / 16 – 192.168.0.0 – 192.168.255.255

These addresses are not routable on the public Internet; they are used behind a NAT (Network Address Translation) device.

6.3 Subnetting & CIDR

Subnetting divides a network into smaller logical segments. CIDR notation combines the address with a prefix length indicating the number of network bits.

IP address:   192.168.1.0/24
Binary:       11000000.10101000.00000001.00000000
Network bits: 24 (11000000.10101000.00000001)
Host bits:    8  (00000000‑11111111)
Network address:   192.168.1.0
Broadcast address: 192.168.1.255
Usable hosts:      192.168.1.1 – 192.168.1.254 (254 hosts)

6.4 Static vs. Dynamic IP Addressing

• Static – Manually configured; remains constant (useful for servers, printers, or devices that need a fixed address).
• Dynamic – Assigned automatically by DHCP (Dynamic Host Configuration Protocol); simplifies management of large LANs.

7. Network Devices, Topologies & Communication Models

7.1 Key Network Devices

7.2 Common LAN/WAN Topologies

• Star – All nodes connect to a central switch or hub (most common in modern LANs).
• Bus – Nodes share a single communication line; historically used with coaxial Ethernet.
• Ring – Each node connects to two neighbours; token‑ring is an example (largely obsolete).
• Mesh – Multiple redundant paths; typical in WAN backbones and some wireless ad‑hoc networks.

7.3 Client‑Server vs. Peer‑to‑Peer (P2P)

• Client‑Server – Centralised server provides resources/services; clients request them (e.g., web browsing, email).
• Peer‑to‑Peer – Every node can act as both client and server; resources are shared directly (e.g., file‑sharing, BitTorrent).

7.4 Thin‑Client vs. Thick‑Client

• Thin‑client – Minimal processing and storage on the local device; most work is performed on a remote server (e.g., web‑based applications, virtual desktop infrastructure).
• Thick‑client (fat client) – Performs most processing locally; requires installation of software and often more powerful hardware (e.g., desktop office suites, video games).

8. Cloud Computing Overview (Syllabus Link)

• Service models

  • Software‑as‑a‑Service (SaaS) – Applications delivered over the Internet (e.g., Google Docs).
  • Platform‑as‑a‑Service (PaaS) – Development platforms provided as a service (e.g., Heroku).
  • Infrastructure‑as‑a‑Service (IaaS) – Virtualised hardware resources (e.g., Amazon EC2).

• Deployment models

  • Public cloud – Services offered to the general public over the Internet.
  • Private cloud – Cloud infrastructure operated solely for one organisation (often on‑premises).
  • Hybrid cloud – Combination of public and private clouds, allowing data and applications to move between them.

• Relevance to networking

  • Relies heavily on DNS for service discovery.
  • Uses virtual networks, load balancers and firewalls to control traffic.
  • Security is enhanced by TLS/HTTPS, DNSSEC and cloud‑provider firewalls.

9. URLs in Other Protocols

10. Key Points to Remember

• A URL tells a browser what to request (host, path, query) and how to request it (scheme/protocol).
• DNS translates the human‑friendly host name into a machine‑readable IP address (IPv4 or IPv6).
• IP addressing, subnetting and the distinction between public and private ranges are essential for routing traffic on the Internet.
• Thin‑client and thick‑client architectures affect where processing occurs and influence bandwidth requirements.
• Cloud‑computing models (SaaS, PaaS, IaaS) rely on the same underlying networking concepts – DNS, IP routing, firewalls and encryption.
• Security measures such as TLS/HTTPS, DNSSEC and firewalls protect data while it travels across the network.