When you change your domain's DNS settings, it takes 2 to 4 days for these updates to propagate across the world's DNS servers. This period is needed for all ISPs to update their DNS records, though some ISPs may delay this by not refreshing their records or clearing their DNS cache in time. The root name servers update their records by querying domain registrars several times daily, and ISPs check these updates, usually within 24 hours. However, some ISPs may take longer, causing users to still access old servers while others have already reached the new server.
DNS caching plays a crucial role during this propagation period. ISPs cache URLs and IP addresses, and if their servers are not configured to clear this cache frequently, users may still be directed to the old server. Internet routers also maintain their DNS cache, but they usually update faster than ISPs.
During this 48-hour (or longer) propagation period, users may face issues such as being redirected to their old server while others can access the new one, fluctuations between the old and new websites, or email problems. These issues arise because the new DNS records haven't fully propagated yet. Once the DNS cache clears, these issues will resolve.