Changes to domain name information in registry-controlled DNS zones are generally not instantaneous.

Once you make changes to your domain with the registrar you purchased your domain from, the changes will first have to be uploaded to the registry, which will then update its DNS servers with the new information. ISPs DNS resolvers can then pick up this information through a sequence of stages, a simplified version of which is as follows:

• the DNS resolver queries the DNS root servers for the registry’s DNS server information
• it then queries the registrar’s DNS servers for the domain’s own DNS server information
• finally, it queries the domain’s own DNS servers (which are typically run either by or on behalf of the domain’s owners, often by the domain registrar that issued domain) for the domain’s detailed DNS information

In practice, this process may involve several extra more cascades of DNS lookups than are described here, but the effect is ultimately roughly the same as the above.

Once an ISP’s DNS resolvers have obtained your DNS record information, they then cache it for a period which may vary from ISP to ISP. This time is generally controlled by the TTL field of each DNS record, but some ISPs may hold the data for a longer or shorter period, depending on the particular software and settings they are using on their DNS resolvers.

As a result, it may take several days for changes to a domain’s DNS records to propagate throughout the Internet; firstly the information must propagate from your registrar to the registry, secondly the registry must update its own DNS servers, and thirdly the ISPs recursive resolvers must forget their cached copies of any previous settings.

Article source is: Wikipedia – Domain Propagation.

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