Understanding what happens during the 3 - 4 days propagation period
When you change your domain DNS settings in your domain registrar control panel , these new settings must propagate throughout the worlds DNS servers.It also means that every ISP ( Internet Service Provider ) must update thier DNS records to reflect these new changes , which in most cases is done automatically every 48 hours, but not always however.
Where do the Root Name Servers receive their information from ?
The root name server will query "domain registrars" several times a day.The root name server will gather this information from the many registrars now in existence and update their master records accordingly.Now your ISP must access ther Root Name servers and update their DNS records which resides on their local DNS server.The process is fully automated and most ISP's will check the Root Name servers for updates every 24 hours.Beware however that some lame ISP's will delay this process for as much as 2 to 4 days in some cases.If that happends, it will no doubt cause additional confusion as everyone else will be reaching your new account on our server except you.This is because your ISP has not updated their DNS records and or have not cleared their DNS cache which means they'll still be pointing your domain name to your old server.If it's a new domain you've registered , then you'll receive a blank 'Site Not Found Page'.
DNS cache and your ISP
Everytime you access a site from your ISP they cache the URL as well as its associated IP number. If their network is properly setup , these DNS cache records should "Expire" at least every 24 hours .If they did not ( which is often the case ) , you'll experience this .You enter your http://www.mydomain.com URL and it keeps taking you back to your old server account.
In a large number of cases, it's the result of an ISP who "Did Not" configure their servers to "Expire" the DNS cache records at the appropriate intervals.Unfortunately,this adds additional confusion to their clients and especially the ones whom are trying to point their domain name to a new server.
The DNS propagation process is not limited to ISP's ! The internet itself must update / clear its DNS cache as well.When we say the Internet,we mean the numerous intermediateÂ "point of access" you're routed through before reaching your final destination.For the most part, these intermediate points of access consist of "Internet Routers" and "Internet Caching Engines".These too,maintain their own DNS cache which assists them in routing traffice / resolving URL's to the correct destination IP's.Don't worry though as Internet routers are usually faster at clearing their DNS cache than ISP's are.
What to Expect during this 2 to 4 day propagation period
In most cases,the propagation process will take at least 48 hours to complete.The first thing that happens is the "World Root Name Servers" will check all lof the various "Domain Registrars for updates".After the root name server has done this processing, the rest of it is up to the many ISP providers who "should be" updating their DNS records ( at least every 24 hours ) ,but a number of them will not.