When you enter a domain name into the address field of your browser, you don't often think about how that domain name connects you to the page that you want to view.
In fact the computers on the internet that serve out the pages of information are not really assigned such meaningful names but instead use IP addressing. An IP address consists of a number consisting of four hexadecimal digits. Each digit of an IP address can have any of 256 different values and we often write IP addresses in a decimal form separating each hexadecimal digit from the next with a dot eg. 10.22.254.117
So how does the domain name that we enter into the address bar of our browser get converted into this IP address that the the computers on the internet can understand$%:
To perform this conversion a number of the computers on the internet act as Domain Name Servers which bind domain names to corresponding IP addresses. Not every DNS can handle converting every domain name into the corresponding IP address, instead the DNS specifies the full conversion only for those IP addresses that are locally hosted and passes requests for other domain names along to another DNS to be processed.
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Let's consider an example that should clarify for you how this works. Let's say that you are in the US or UK and are enter into the address bar of your browser. This domain is not hosted locally (its hosted in Australia) so your ISP's DNS does not have an entry to translate the full address entered into an IP address. Instead it has an entry for the au domain that points at another DNS. The request is passed to this new domain name server which now has an entry for asn.au which passes the request through to yet another DNS. Finally the request reaches a DNS that has an entry that binds the domain name entered to the appropriate IP address and is able to forward the request to the server hosting the requested web page. If you are in Australia and enter this address then your local DNS may or may not have an entry for this domain and if not the request again gets passed to another DNS for processing.
This process is normally transparent except for when a site changes web hosts. When a web site is moved, the domain name servers that have an entry binding the name to an IP address need to be changed to reflect the new IP address. The new host updates their DNS which passes information to the domain name servers at a higher level to advise them that it is now the master DNS for that site. At regular intervals the various Domain Name Servers interrogate one another to see what domain names have changed address. Until such time as all of the DNS that have an entry relating to a given site have updated themselves to reflect such a move, requests that pass through a DNS that has not been updated will be passed to the old IP address instead of the new one while requests that pass through a DNS that has been updated will go to the new host. This means that, where possible, a copy of the site should be maintained with the old host for several weeks after moving your site to a new host so as to allow time for the change of IP address to trickle through to all of the relevant DNS. Where this is not possible to do, you may end up with a situation where some potential visitors to your site receive a "Page not Found" message because their request went via a DNS that has not been updated yet while the rest of your visitors are still able to access your site because their request goes via a DNS that has been updated.
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