Question

I hosted my website using appengine ,after I change the subdomain on which it was hosted to another subdomain,It took some hours to again go live.Why is it so,and what takes the time?

Answer 1

http://support.powerdnn.com/KB/a604/dns-propagation-and-why-it-takes-so-long-explained.aspx

Why Does DNS Take So Long to Propagate? You have registered your
domain name, uploaded your website to one of our web servers, and
asked your registrar to either use our name servers or to point your
“A” record to your web server’s IP address. Once that this is done,
what’s the hold up?

When your website’s address is entered into a browser, the computer
requests the IP address of the server housing your site from your
Internet Server Providers (ISP) DNS records. If the site is not listed
in the records it queries registrars to find out who the DNS start of
authority (SOA) is for your website. If you’re using your registrar’s
name server as your SOA, it looks up the “A” record for your domain
and returns the IP address of the server listed. If you are using our
name servers, the registrar points the browser to our DNS servers to
determine the IP Address for your domain name. From there the request
is sent to the server the domain is hosted on which then provides the
browser with the website.

To speed the loading of websites, each ISP caches a copy of DNS
records for a period of time, sometimes up to 48 hours. This means
that they make their own copy of the registrars’ master DNS records,
and reads from them locally instead of making a direct request to the
domain registrar every time a request for your site is made. This
speeds up web surfing quite a bit by: decreasing the return time it
takes for a web browser to request a domain lookup and get an answer
and reducing the amount of traffic on the web. The downside to
caching the master DNS records is because each company or ISP only
updates their records every few days, any changes you make to your DNS
records are not reflected between those updates. Although our DNS
servers update every 15 minutes, the time between updates system wide
is not standardized so the delay can range from a few hours to several
days. This slow updating of the cached records is called propagation
delay because your website’s DNS information is being propagated
across all DNS servers on the web. Once completed, everyone can visit
your new website.