N&SA-2025-S1-Wk3-2

Domains and Subdomains

• Domain Hierarchy: A domain can have multiple subdomains, creating a hierarchical structure.

  • acmebw.com is the main domain.
  • Subdomains like sales.acmebw.com exist.
  • Domains can extend to the fourth level or more, each being unique.

• Subdomain Definition: A domain is a subdomain of another if its root node is a descendant of the other's root node.

  • Example: sales is a subdomain of acmebw.com.

• Example Breakdown: cannabis.uti.82.ac.nz illustrates subdomain structure.

  • cannabis is a subdomain of uti.82.ac.nz.

• Email Access Example: Accessing email might involve a subdomain.

  • mail.82.ac.nz or staff.82.ac.nz, where mail or staff is the subdomain.
  • The structure is: subdomain (level four) to level three, to level two, up to the main domain.

Root of DNS

• Coherent Namespace: The root of the Domain Name System (DNS) provides a coherent, consistent namespace using a single root in a hierarchical tree structure.
• Uniqueness: This root holds definitions for all top-level domains, ensuring each is unique.
  • There can only be ONE, guaranteeing uniqueness.
• Inconsistency Issues: Violating this rule leads to inconsistency, potentially directing users to the wrong domain.
  • This is a method criminals use to steal information.

Domain Registration

• Guarantee of Uniqueness: Registering a domain name properly ensures that only one exists, and it belongs to the registrant.

DNS Name Servers

• Limited Number: Due to protocol limitations, there are only 13 root name servers worldwide.
  • Their primary job is to manage the root.

RFCs and Top-Level Domain Structure

• RFC 881 and RFC 920: These documents detail the concept of functional domains.

  • .com for commercial enterprises.
  • .edu for education.
  • .gov for government.

• RFC 920: It also provided for country domains.

• Stability: The top-level domain structure remained stable until around 1997 (covered later).

Current Top-Level Domains

• Generic Domains:

  • .com
  • .net
  • .org

• Country Codes: Top-level domains include country codes.

  • .af for Afghanistan
  • .nz for New Zealand
  • .eu for Australia
  • .al for Albania
  • .dz for Algeria

• Domain International: For international treaty organizations.

U.S. Legacy Top-Level Domains

• Governmental Organizations: .gov
• Military: .mil
• Educational Institutions: .edu

Country-Specific Government Domains

• Specific Setups: Countries like Australia and New Zealand use specific setups to denote government domains.
  • New Zealand Government: .govt.nz
  • Australia: .gov.au