In order to understand domain propagation, you must first understand a little about how DNS works. When you register a domain and secure a hosting account, the DNS point all aspects of that domain to where they find the information to load various services. MX records control the functionality of your email. CNAME records control different aliases of your domain. A records control the IP associated with your domain. Nameservers direct your domain to the server your hosting account is housed on. Your domain is the vehicle and your domain’s DNS is the roadmap to get it, and all its cargo, to the intended destination point(s).
When you change individual records or nameservers, your domain registrar (the company you paid to register ownership of your domain name) updates that information across the internet to reroute that traffic to the new appropriate place. The time between when the change was made and when the domain is actually rerouted is called the “propagation” period.
Each ISP (internet service provider) caches their DNS records in order to speed up the transfer of that data. In addition, your registrar also sets “kill time” (also called TLL or time-to-live) which dictates how often they report changes to your DNS. Caching is basically storing LOCAL copies of those records so that when they are accessed, the local version is transmitted rather than taking the time to re-pull the data from the internet every time a website is loaded in order to make the return time faster when a web browser “calls” for a domain and to reduce the traffic on the web. Since these two forces can work MUCH differently, changes in DNS can get hung up in cyber space for days in some cases.
Since there is no real standard for caching, some set the kill/expiry time to 14400 seconds (4 hours) while others can range up to 345600 seconds (96 hours = 4 days!). When this propagation period has finalized, everyone will be able to access your website at the new location. Because of the difference in cache time between ISPs and registrars, the update process (propagation) happens at different times in each geographical location so even though someone in Russia or California may see things based on the updated information, someone in Atlanta or Malaysia may still be seeing things based on cached data.
This process, on average, takes anywhere from 4 to 48 hours to complete, but it can sometimes last for 72 or even 96 hours (depending on individual registrars and ISPs) for DNS changes to fully propagate in all areas.