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Lesson 10 Domain Name Service Structure
ObjectiveDescribe Internet Domain Names

Domain Name Service Structure

The Domain Name Service Structure (DNS) serves several critical purposes:
  1. Human-Friendly Internet Navigation:
    • Domain Names: DNS allows us to use memorable domain names (like []( instead of difficult-to-remember numerical IP addresses (like
    • Translation: DNS servers act like giant phonebooks, translating domain names into their corresponding IP addresses, which computers use to locate and communicate with each other.
  2. Decentralized and Resilient System:
    • Hierarchical Structure: DNS is organized in a tree-like hierarchy. This distributed design prevents a single point of failure, making the internet more reliable.
    • Authority Delegation: Responsibility for portions of the domain name space is delegated to different name servers. This distribution spreads the load and improves response times.
  3. Resource Mapping:
    • Beyond Websites: DNS maps domain names to various resources, not just web addresses. This includes:
      • Mail servers (for email routing)
      • File servers (for file sharing)
      • Other services connected to the internet
Key Points to Summarize
  • Translation: Turns easy-to-remember names into IP addresses computers understand.
  • Decentralization: Makes the internet more robust and prevents a single bottleneck.
  • Resource Mapping: Helps locate various types of services on the internet.

Hierarchical Tree Structure

Domain names follow a strict naming convention with a three-level hierarchy:
  1. root level,
  2. top level, and
  3. second level.
Each part of the name is separated with a period, the "dot" you hear when someone says a Web address out loud. The figure below illustrates the domain name hierarchy.
Domain Name Hierarchy consisting of 1) Root Level, 2) Top Level, 3) Second Level
Domain Name Hierarchy consisting of 1) Root Level, 2) Top Level, 3) Second Level

Root-level Domains

The root-level domain is the starting point in the hierarchy.
  • Top-level domains: There are two types of top-level domains: original and country. The table below lists the original domain names and the types of organizations to which they are assigned.

Zone Definition For Use By
.govGovernmentU.S. federal government agencies
.intInternationalOrganizations established by international treaties
.milMilitaryU.S. military
.netNetworkNetwork Providers, administrator computers, network node computers
.orgOrganizationNon-profit and miscellaneous organizations

Country-level domains are called country code top-level domains (ccTLDs). They correspond to a country, territory, or other geographic location. xamples are us (United States), uk (United Kingdom), de (Germany), and jp (Japan). The list of valid domain names is constantly being revised.


In addition to a country-level domain, the us domain is further divided into subdomains, with one subdomain for each state and one for Washington, D.C. The state subdomains are further divided into cities, counties, or other regional groupings. For example, is Cleveland, OH; is San Francisco, CA. While most private domains does not utilize this tedious naming convention, many government agencies do. The trend, however, is toward the simpler dot-origin name method for these government Web sites.
  • Second-level domains: Here's an example to illustrate how second-level domain names can contain both hosts and other domains called subdomains, using the domain name "":
    1. Second-level domain name:
      This is the main domain name registered under the top-level domain .com.
    2. Subdomains:
      • This could be a subdomain specifically for hosting a blog related to SEO and digital marketing topics.
      • This subdomain might be used for an e-commerce section where SEO tools or services are sold.
      • A subdomain dedicated to customer support and help resources for users of services.
    3. Hosts:
      • A host under the domain, typically used for email services.
      • This host could be set up for file transfer protocols, allowing files to be uploaded or downloaded.

    In this example, "" serves as the second-level domain name under which various subdomains and hosts can be organized. Each subdomain can effectively function as a distinct branch or section of the main domain, serving different purposes and hosting different types of content.

TCP, UDP, SCTP Transport Layer

When an application requests services from TCP, UDP, or SCTP at the transport layer, it must supply a numerical IP address. TCP/IP provides a support application that fulfills the role of the Domain Name System (DNS) protocol, to translate domain names into IP addresses. The DNS application uses a massive distributed database organized as a directory system of servers to obtain the required information. Each entry in the database consists of a domain name and an associated IP address.
The directory system that is used to translate domain names into IP addresses is organized as a tree structure, very similar to the directory structure of a computer operating system, except that there is a separate server at each node on the tree. Figure 4-10 shows the structure of the tree. Each directory node on the tree provides name-to-IP address services corresponding to its position on the tree. There are three primary levels of interest. Below that, individual domain name owners can extend the number of levels down as far as they wish for convenience of organization and clarity.
Domain Name System Server Hierarchy
Figure 4-10: Domain Name System Server Hierarchy

DNSroot Server

At the top of the tree is the root directory, called the DNSroot server. Actually, there are thirteen of these servers, scattered all over the world, and each of them is a cluster consisting of many computers. The DNS root servers must handle a large number of queries, more than 50 billion a day as of June 2009. Disbursing the root servers geographically reduces the amount of long distance traffic by providing nearby access for as many queries as possible. The DNS root servers have entries for all of the so-called top-level domains. There are country-code top-level domain name servers (ccTLDs) for every identifiable country in the world plus a number of authorized commercial and noncommercial type domains. The non-country-code domains include .com, .edu, .org, .net, and many more.
In mid-2008, a decision was made to allow the creation of additional top-level domains. As of June, 2008, there were more than 160 million domain name registrations, according to the Domain Name Industry Brief .Domain names below the top-level domains are registered for a small fee by users with one of a number of registrars. ICANN assumes overall responsibility for the millions of registered names on the Internet. Domain names at this level are called local domains. The name must, of course, be unique; there can be no duplicates anywhere in the world. Once the domain name is registered, it can be used to assign names of individual nodes or hosts within that domain, and matched to one or more assgned IP addresses. Domain names are read left to right, from the lowest subdomain to the top level domain.
In the next lesson, you will learn about the functions, components, and types of URLs.

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