Each Internet Server Provider caches their DNS records.
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.
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