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• Internet Addressing
• Why domain names?
• What is DNS?
• The Internet Domain Name Structure
• How to read a domain name
• Valid domain names
• How does DNS work?
• What are name servers?

Internet Addressing
The Internet uses two addressing schemes--number-based Internet Protocol (IP) addresses and text-based Domain Name System (DNS) names. An IP address consists of four numbers ranging from 0 to 255, separated by periods. Each machine on the Internet must have a unique IP address. A domain name is a textual address for a location on the Internet. It corresponds to the actual alpha numeric address which the Internet computers can read.

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Why domain names?
Although IP addresses provide a convenient, compact representation for locating machines, on the internet, users prefer to assign machines pronounceable, easily remembered names. A domain name can also create an expectation about who is at that location because it is often a shorthand naming term, or even a trademark or descriptive term with secondary meaning. Further more, the use of domain names has the advantage of shielding network changes from users. Names are often given to machines according to the services that they offer. For example, www normally denotes the name of the web server on the network. Users remember such names rather than specific IP address. In the event that the web server is shifted from one machine to another, only the IP addresses change but not the name. Hence, users can still access the web server using the same name and the change in machines is transparent to the users.

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What is DNS?
The Internet Domain Name System ( DNS ) is a set of distributed databases containing IP addresses and their corresponding domain names. Each domain name is mapped to a particular numeric address. DNS, with servers located all over the Internet, performs the translation back and forth between names and numbers. This scheme enables users to invoke an easy-to-remember name ( e.g. www.nic.net.sg) instead of a more mysterious string of numbers ( e.g. 160.96.7.25) when using the Internet. In the beginning, the original set of machine names used throughout the Internet formed a flat name space. A central site, the Internet Network Information Center ( NIC ), adminstered the namespace and determined whether a new name was appropriate ( i.e. it prohibits obscene names or names that conflicted with existing ones ). As the internet grew, this system was no longer appropriate. Hence, a hierarchical naming scheme is used and the name system is decentralised by delegating authority for parts of the namespace and distributing responsibility for the mapping between names and addresses.

Further Readings on DNS and MX records :
(YOU WILL LEAVE THIS SITE)

The Domain Name System. An extensive DNS Guide written by Glenn Stevens.

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The Internet Domain Name Structure
The Internet Corporation of Assigned Names and Numbers (ICANN) is responsible for managing and coordinating the DNS. The DNS top level name space is divided into 2 broad categories, the GTLD(General Top level Domains) and ccTLD(Country-Code Top Level Domains). Examples of GTLDs are (.com, .net, .org, .mil, .name, .info, .aero etc).

ccTLDs have been typically assigned to national, or regional registries. To further classify and divide the name space, some national registry have created subdomains under the country code TLDs.

In Singapore, SGNIC is the authority that adminsters the name space for the .SG domain. The .SG domain is further subdivided to com.sg, net.sg, org.sg, gov.sg and edu.sg. Please see domains under SG for the requirements to register under these second-level domains.

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How to read a domain name
A simple domain name,

           www.myexample.com.sg,

is read from left to right (most specific to least specific), the first item (www) is the name of the host computer itself, or the hostname. The next item (myexample) is the third-level domain name, and is registered by the organisation with SGNIC. The second last item (com) is a second-level domain name, and describes the purpose of the organization who owns the third-level name. A domain name may include other components between the hostname and the second-level domain name; these are called subdomains.

Definition of FQDN :
A Fully Qualified Domain Name (FQDN) is a domain name that includes all higher level domains relevant to the entity named. For a host, a FQDN would include the string that identifies the particular host, plus all domains of which the host is a part up to and including the top-level domain. For example, www.nic.net.sg is a FQDN of the SGNIC web server at 160.96.7.25.

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Valid Domain Names
Domain names can only contain alphabets, numbers and dashes (i.e. hyphens or the minus signs). The underscore character, "_", is an invalid character. SGNIC will reject applications for domain names containing invalid characters. Each field in the domain name should be less than sixty three characters long. Applications for long domain names such as very-long-domain-name-with-more-than-63-characters-in-total-for-3rd-level-only.COM.SG will be rejected.

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How does DNS work ? - A simplified example
Let's suppose that a user is accessing this web site, www.myexample.com.sg, for the first time. Here are the sequence of steps :

1.	Browser machine sends a request to the local name server to resolve the IP address of the web site.
2.	The local name server does not have the domain name in its cache. It issues a request to the SG name server, maintained by SGNIC.
3.	The SG name server looks up its table and returns the primary name server responsible for myexample.com.sg (and its secondary name servers).
4.	The local name server contacts the primary name server.
5.	The primary name server looks up its database table for myexample.com.sg and returns the ip address of www.myexample.com.sg.
6.	The local name server returns the IP address to the browser.
7.	The browser contacts the web site.

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What are name servers?
Name servers are computers that map easily forgettable numbers that computers understand, into easily remembered names.

For each domain name, you need at least two DNS (Domain Name System) servers - a primary and at least, one secondary DNS server, as required by Internet guidelines (RFCs 1591 & 2182. The name servers must be continuously connected on the Internet but need not be dedicated i.e. they can run on servers performing other work (e.g. mailservers, webservers etc). You can also use one name server system for more than one domain. If you do not have your own dedicated server, your local ISPs or domain name registrar can usually provide DNS hosting name services. However, such services are normally not free.

The physical location of your servers does not matter. Your organisation can register for a domain name as long as it satisfies the requirements.

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