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Lesson 1

Domain Name Service

The Domain Name Service (DNS) is an Internet-wide service for converting numeric IP addresses to host names and back. It may also be used on networks without an Internet connection; indeed, setting up DNS is a worthwhile investment, because it simplifies integrating a local network into the Internet when a connection becomes available.
By the end of this module, you will be able to:

  1. Describe the organization of the DNS namespace
  2. Define the top-level subdomains of the DNS
  3. Describe the process of converting IP addresses into names
  4. Define the concept of zones as used in the DNS namespace
  5. Describe the three classes of name servers
  6. List the steps required when a name server sends a query up the DNS hierarchy
  7. Describe the purpose and usefulness of caching-only name servers
  8. Identify the types of Resource Records
  9. Describe the general format of a zone file
  10. Describe the format of SOA, A, PTR, HINFO, CNAME, and NS records



Billions of hosts are connected to the Internet.
Question: How do we keep track of them all when they belong to so many different countries, networks, and administrative groups?
Two key pieces of infrastructure hold everything together:
  1. the Domain Name System (DNS), which keeps track of who the hosts are, and
  2. the Internet routing system, which keeps track of how they are connected.

Although DNS has come to serve several different purposes, its primary job is to map between hostnames and IP addresses. Users and user-level programs like to refer to machines by name, but low-level network software understands only IP addresses (that is, numbers). DNS provides the glue that makes everything work.
It has also come to play an essential role in the routing of email, web server access, and many other services.
DNS is a distributed database. Distributed means that my site stores the data about my computers, your site stores the data about your computers, and our sites cooperate and share data when one site needs to look up the other's data. From an administrative point of view, your DNS servers answer queries from the outside world about names in your domain, and they query other domain servers on behalf of your users.
Domain Name Servers (DNS) are the Internet's equivalent of a phone book. The DNS maintains a directory of domain names and translates them to (IP) Internet Protocol addresses. This is necessary because, although domain names are easy for people to remember, computers or machines, access websites by means of IP addresses.
Information from all the domain name servers across the Internet are gathered together and housed at the Central Registry. Host companies and Internet Service Providers interact with the Central Registry on a regular schedule to get updated DNS information. When you type in a web address, i.e., www.distributednetworks.com, your Internet Service Provider
  1. views the DNS associated with the domain name,
  2. translates it into a machine friendly IP address and
  3. directs your Internet connection to the correct website.
After you register a new domain name or when you update the DNS servers on your domain name, it usually takes about 12-24 hours for the domain name servers world-wide to be updated and able to access the information. This 24-hour period is referred to as the propagation period.