Web Structure and DNS

Jun 14, 2025

Overview

This lecture explains how the World Wide Web works by breaking down URLs, domain names, IP addresses, and the domain name system (DNS), including the roles of Internet registrars and domain name servers.

Understanding URLs and Domain Names

  • A URL (Uniform Resource Locator) is the full web address used to access a resource on the internet.
  • The domain name is the website's main identifier, like "bbc.co.uk" or "amazon.com".
  • Adding a host prefix (e.g., "www") creates a Fully Qualified Domain Name (FQDN).
  • The URL includes protocol (HTTP/HTTPS), host, domain, path, and resource file.

Structure and Organization of Domain Names

  • Domain names are organized in a hierarchy, written from specific (left) to general (right) using dots.
  • Third-level domains are most specific (e.g., "bbc" in "bbc.co.uk").
  • Second-level and top-level domains follow, with top-level domains like ".uk", ".fr", ".com", ".org", or ".net".

Internet Registrars and Domain Name Management

  • Domain names are managed by global organizations called Internet registrars, overseen by ICANN.
  • Registrars maintain databases to ensure each domain name is unique and not reused.

IP Addresses and Their Connection to Domain Names

  • Every device on the internet has a unique IP address, like "193.60.99.173".
  • IP addresses allow network devices to locate each other but are harder for humans to remember than domain names.
  • There are different IP address versions (IPv4, commonly four numbers; IPv6, more complex).

The Domain Name System (DNS)

  • DNS translates human-friendly domain names into machine-friendly IP addresses.
  • When a URL is entered, the browser extracts the FQDN and asks the local DNS server for its IP address.
  • If the local DNS server lacks the mapping, it queries higher-level DNS servers until the address is found.
  • Once found, the mapping is cached for quicker future access.
  • The browser then uses the IP to request the website’s resources.

Key Terms & Definitions

  • URL (Uniform Resource Locator) — The complete address used to access a resource on the web.
  • Domain Name — The readable web address for a site (e.g., "amazon.com").
  • Fully Qualified Domain Name (FQDN) — Domain name including host prefix (e.g., "www.amazon.com").
  • IP Address — A unique numerical label for each device on a network.
  • DNS (Domain Name System) — The system that matches domain names to IP addresses.
  • Internet Registrar — Organization managing domain name registration and uniqueness.
  • ICANN — The global body overseeing Internet registrars.

Action Items / Next Steps

  • Review the hierarchy of domain names and their components.
  • Understand how DNS servers resolve domain names to IP addresses.
  • Prepare for upcoming material on IP version differences and network protocols.

Full transcript