DNS propagation is a term used to describe the time-frame after making any changes to your domain name.
Such propagation will occur after any:
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Changes to your domain's WHOIS information.
WHOIS is a publicly available tool that searches the databases of domain registries and registrars to detail the domain owner contact information. A change to your WHOIS information (such as updating your contact details) can take up to 72 hours to propagate fully.
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Changes to the DNS records or name servers of your domain.
Every time you use a domain name on your computer its DNS information will be stored in a caching system. This can be local cache on your computer's operating system, DNS cache stored by your Internet service provider, etc.
DNS records are stored in cache mainly to improve the performance of DNS queries. Every DNS record has a Time to Live value, which is the time DNS servers should store that record in cache. Even if a record is changed, DNS servers will continue working with its formal value from cache until this time has passed.
This is the essence of DNS propagation - it is the time required for DNS servers worldwide to update their cached information for a domain name. It is influenced by the TTL of DNS records that might have changed, but there are also other factors that could come into play.
A DNS change requires up to 72 hours to propagate worldwide, although most often this happens in a matter of hours.
