When you update the nameservers for a domain, it may take up to 24-72 hours for the change to take effect. This period is called DNS propagation. In other words, it is a period of time ISP (Internet service provider) nodes across the world take to update their caches with the new DNS information of your domain.
Due to DNS caches of different levels, after the nameservers change, some of your visitors might still be directed to your old server for some time, whereas others can see the website from the new server shortly after the change.
Why DNS propagation can take up to 72 hours?
Let’s imagine - you change the nameserver for your domain hosted in a country that is different from the one in which you live . When you open your domain in a web browser, your request is not going to the hosting server directly, it has to pass through several ISP nodes first. So your computer starts by checking local DNS cache, then the request is sent to your local living country ISP. Each of the ISP nodes checks its own cache to see if it contains the DNS information of the domain. If it is not there, it looks it up and saves it in order to speed up the loading next time and to reduce the traffic.
Basic methods for the reducing time of DNS propagation.
There are three basic methods that will allow you to pass the DNS propagation. If you have not changed the nameservers and do not wish them to propagate for so long, there is a way to reduce the propagation time. You need to do two simple things:
1. Point your domain to the destination IP address by means of A record on the side of the current DNS provider, setting the minimal TTL ('Time to live' – propagation time) for this record, for instance, to 300 seconds (5 minutes).
2. Once A record has been updated, wait up to 30 minutes and change the nameservers for your domain.
As a result, your domain will be resolved to your previous host from the places where the propagation has not been completed yet and to a new one – from the places where it has already passed. In such a way, you may avoid a downtime as both hosts will show you the same result – your new website.
If you have already changed the nameservers, Google public DNS tools may help you to see your website online. Here are the steps to be followed:
1. Set Google Public DNS servers.
2. Once done, clear your browser's cache and flush your local DNS cache.
3. In addition, you can use Google Flush Cache tool and flush NS and A record for your domain name.