What is mean by Propagation delay ?
Lets look close into it:-
When a request to get the IP of a domain is generated from a machine or client, the ISP fetches the information from the authoritative name servers. Then it keeps the records in the ISP cache for a
specified interval of time. This process of keeping the records is called ISP caching.
It can be compared with a browser cache up to a good extent. If another user requests to access the domain
from the same ISP, the IP stored in the cache will be served to the new client. Obviously the speed will be higher. You can experience the difference while accessing google.com and a normal, not so common domain.
Unlike browser cache, we don't have any control over ISP cache and can't delete the cache file. So when a change is done on the authorative DNS servers, to reflect it on all ISPs we need to wait for the refresh interval specified in zone file. This time delay can take up to 48 hours and is called DNS propagation delay.
During this time, there can be difference for the modified records from different ISPs.
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