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Lesson 9Domain names
ObjectiveDescribe the Domain Name Service (DNS)

Domain Name Service (DNS)

Internet domain names

Domain names are alpha strings used to identify a Web site. For example,, or are domain names. However, the Internet truly identifies Web sites by its IP address (example: 334.483.45.12). So, each time a user types a domain name into their browser's and presses Enter, the domain name is sent to the Domain Name Server (DNS) for translation to its assigned numeric IP address.
Domain Name Services (DNS) are dedicated servers (also called a DNS: Domain Name Server) which hold a two-column list of all the domains and IPs on the Internet (and, additional information, as we will see later in this module). For the purposes of explanation, the list has been simplified below:

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

When you want to launch a Web site, you must register a domain and host it with a provider. Registration requires two things: selecting a name (such as and the IP address to the Host site (the company's computer where the Web site will be stored). Once known, the registrar (Internic) broadcasts that information to every DNS on the planet.
Within about 24 hours, all DNS servers know the IP address to

DNS server types

Because every domain name request must be translated to an IP address, thousands of DNSs are used on the Internet. Since DNSs are dedicated computers, they tend to be costly. To pay for these costs, larger organizations sell access to their DNS. Since DNS is critical to the operation of the Internet providers tend to rent services from two DNSs for redundacy. The two DNSs used are referred to as primary and the secondary, and each one is designated by its IP address (not its domain name, as that would slow things down).
For efficiency DNSs use a tiered approach when translating domain names. Smaller DNSs provide translation to a limited number of local providers (ISPs) and offer maximum speed response by only storing the domain names (and IPs) of only the most recently requested addresses. When a domain outside the list is requested, the local DNS makes a request to a larger regional DNS for the IP. If the regional DNS does not have it, the local DNS sends a request to an even larger DNS until the domain is located. Now you know why it sometimes takes a long time to display a site. It may be that your local or regional DNS did not have the domain name in its list.

DNS record types

DNS stores its information in tables, and each table is composed of records. Each record is composed of a host name, a record type, and an address.
In the next lesson, you will learn about the DNS structure.