Understanding DNS
If you understand the basics of IP addresses and TCP/IP, the remaining idea required to get a grasp on the basics of the Internet is the DNS. DNS is an abbreviation for Domain Name System, but it can also mean Domain Name Server, depending on the context in which it is used.
DNS is an heirarchical naming system that allows computers to be defined and located through user-friendly names rather than through hard-to-remember IP numbers. The DNS system does this through resolving DNS addresses to their IP number equivalent. It was devised by Paul Mockapetris, then a member of the IETF.
When a user enters a website address in their browser, the DNS sytem rapidly matches the registered domain name, say www.netcomm.com.au, with a matching IP address, say 216.218.210.207, through a procedure called DNS lookup. The IP address it looks up will usually be that of a computer server which responds by sending Web page information through the HTTP (Hypertext Transfer Protocol) protocol. Such servers are generally called webservers.
What actually happens when the user enters the address is that the user's computer 'asks' the nearest name-server 'What IP address corresponds with this Domain Name?' If the computer that is asked does not know, it passes the query up to the next name server, until it reaches an 'authoritative source' which is a computer that knows without having to ask. The ultimate authorities are the Root Servers, mentioned below.
Although this is generally quick, it can sometimes take 15-20 seconds to resolve an address request, during which time the browser will display an 'attempting to find address' message; any longer than this and the web browser is likely to give up and declare the address unreachable.
How DNS is co-ordinated and maintained
Maintenance and administration of the DNS falls under the responsibility of ICANN, the Internet Corporation for Assigned Names and Numbers. ICANN in turn devolves responsibilities for the sale and registration of Domain Names to accredited registrars, a list of which is here. If one wishes to register a domain name, these are the companies to which application must be made.
For the DNS system to work, a current list of all registered domain names must be accessible to any computer connected to the Internet. This bit of Internet wizardry is accomplished by distributing copies of DNS records to 13 'root-servers' located at various points around the planet. The data contained in these records is then propagated throughout the Web and is 'cached', or kept in the short-term memory, of computers at Internet Service Providers worldwide. A variety of other techniques are used to ensure that DNS lookups happen very quickly from your average desktop.
Incidentally, every shipping PC has the root-server information built into it at the factory. This is one of the reasons why the Internet works so seamlessly.
As an illustration of the way the DNS works, note that when you register a domain name or move it to a different hosting service, it takes 24-72 hours for the change to ripple through the network of Root Servers and thence through the remaining records on the Internet.
Components of a Domain Name
We have seen that when we enter a web address, the DNS matches the request with the IP address of a Webserver; if it can't make the match, it will return a 'can't find server' error message.
Let's look at elements of a web address in more detail.
Top Level Domains
The web address frequently ends with a gLTD, or Generic Top Level Domain. This is an extension, such as .com, .org, .net. and so on, which loosely describes the kind of organisation that the website belongs to. These are summarised as follows:
| .aero | air transport industry |
| .biz | business use |
| .com | general commecial |
| .coop | co-operative associations |
| .info | Informational resource sites |
| .museum | museums |
| .name | individuals |
| .net | various networks including some some broadcasters [e.g.www. abc.net.au] |
| .org | intended for non-commercial organisations |
| .pro | credentialed professionals and professional bodies |
| .gov | Government exclusive |
| .edu | accredited educational institutions |
| .mil | US Military |
| .int | registered organisations established by treaty |
Some of these extensions are rarely encountered; almost every site encountered is likely to be .com, .net., .org, and .gov with .biz sites seen from time to time. It is interesting to note that, for some reason, in the UK, .com sites generally have the gLTD of '.co'.
Country Codes
Many web addresses have an additional TLD code which signifies their geographical location, for example, .au for Australia, .uk for Britian, .ca for Canada, and so on; a complete list is here. Note that the gLTD '.com' can be registered anywhere in the world; it is a common misconception that '.com' sites must be registered in the US. In fact in the early days of the Web, any site with no country indicator could be assumed to be a US site, but since 2002 the TLD '.us' has existed to denote US websites, although it is not often seen.
Second-Level Domain Name
The second-level domain name comprises the unique identifier that describes the company, organisation or network that owns the domain, for example 'NetComm'.
Subdomains
You might have different subdomains, indicated by the left-most sequence of letters in a Web address. The default value is 'www' however this is often changed to denote a particular function or division with a domain, for example <support.companyname.com> for company support information, or the site hosting this document which is <lc.netcomm.com.au>. Subdomains also allow companies to run diverse kinds of webservers utilising different server technologies under the umbrella of one domain name, among other things.
Hostnames
Finally a web address might include a hostname which is the name of a specific computer on a network. A Fully Qualified Domain Name will include full descriptions of all of these elements of a Domain Name.
Next we will look at some other important aspects of understanding DNS.