It’s important to note that since it can take several hours for DNS records to resolve in all locations, different computers in different locations see the update at different times.
Many people incorrectly refer to a mysterious 48 hour or 72 hour propagation time when you make a DNS change.
The period of validity may vary from just seconds to days or even weeks.
For example, the TTL on Dream Host’s server is set to 4 hours by default.
The Time-To-Live (TTL) value for a domain is how long a website is cached (stored in memory) on a web server. If you are moving a domain from one hosting provider to another, it may be a good idea to drop the TTL as low as possible BEFORE you initiate the migration.
This allows your DNS records at your current host to expire faster so you do not need to wait the full 4 hours until the records change.
Many DNS administrators find that the Dynamic DNS update process, and aging/scavenging processes may be difficult to understand and manage correctly.
In fact, DNS servers check in with your local DNS server as needed, and the administrator of your local DNS server controls the 'time to live' (TTL) values for all DNS records in your domain.
How long a resolver caches a DNS response (i.e., how long a DNS response remains ) is determined by a value called the time to live (TTL).
The TTL is set by the administrator of the DNS server handing out the response.
This page provides details on DNS Propagation, DNS Caching, TTL, Flushing DNS cache, and tools for checking propagation status.
Because of the large volume of requests generated by a system like DNS, the designers wished to provide a mechanism to reduce the load on individual DNS servers.