This question gets asked quite alot and many fail to understand what it actually means.
What is DNS Propagation?
Before trying to understand what DNS Propagation is, one should know the terms involved.
DNS - Domain Name System. When you enter a webpage name, the computer internet network should identify from which server it should get the page’s content from. This is like referencing a phone book. The network doesn’t work with the website name but rather it's associated IP address. The DNS has redirection links from the meaningful site names to network recognizable IP address.
Domain Registrar Control Panel - These are the agents with whom one registers their domain name. They provide a control panel where one can change their DNS settings.
Worlds DNS Servers - There cannot be a single DNS to maintain all the requests in this world so there are numerous servers which refer to Worlds DNS Servers.
Root Name Servers - Currently there are 13 well known Root Name Servers that implement the root namespace domain for the Internet's official global implementation of the Domain Name System. These are nothing but DNS Servers which are the officially declared global name servers.
ISP - Internet Service Provider. These are the organizations which provide internet service to you.
When you change the DNS setting in your Domain Registrar Control Panel, that setting information should be passed on to all the DNS servers in the world which is eventually used to update the DNS servers of the ISP (Internet Service Provider). But this takes time, so there is a delay from the time you actually change the setting and that being effected throughout the world. This is known as DNS Propagation.
Detailed Explanation: Why there is DNS Propagation Delay?
The Root Name Servers are the ones which have the correct information regarding the DNS setting of all websites at any point of time. To maintain their accuracy, they constantly query ‘Domain Registrars’ throughout the day. When anybody changes the DNS setting, then it is being passed onto the Root Name Servers. All the major DNS servers get it updated with the information from Root Name servers.
In order to speed up the rate at which their customers can view the internet, each ISP caches their DNS records. This means that they make their own copy of the master records, and read from them locally instead of looking them up on the Internet each time someone wants to view a website. This actually speeds up web surfing quite a bit, by (1) speeding up the return time it takes for a web browser to request a domain lookup and get an answer, and (2) actually reducing the amount of traffic on the web therefore giving it the ability to work faster..
These caches should be set to expire in a stipulated time so that it refreshes with the latest information from DNS, but the downside to this caching scenario and what makes it take so long for your website to be visible to everyone is that each company or ISP that caches DNS records only updates them every few days. This is not any kind of standard and they can set this time anywhere from a few hours to several days. The slow updating of the servers cache is called propagation, since your websites DNS information is now being propagated across all DNS servers on the web.
When this is finally complete, everyone can now visit your new website. Being that the cache time is different for all servers, as mentioned above, it can take anywhere from 36 to 72 hours for DNS changes to be totally in effect, until then it will show the contents from the old server rather than the new server. Sometimes there may be occurrences where you are able to view the site with the updated setting from the office but not from home, this maybe because the office ISP has updated their local DNS but the home ISP has not updated it's DNS. There may also be weird cases where your site is swinging between the old server and new server in the same ISP, this means that the ISP cache is getting updated. There may also be instances where the site becomes unavailable for a period of time while the new DNS is picked up/updated by the varying ISP's around the world. Some ISP's are notably slower than others.
If it takes more than 3 - 4 days, you might have a problem with the new hosts server DNS setting and you have to contact them to rectify it.
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