Just taking an application offline does not necessarily mean
that the site is down. But in order to inform the user, a proper method
selection is required when we take our application offline. Out of the
different techniques mentioned above for taking a web application offline, the
most reliable way is from IIS, since that guarantees that all requests will be
either rejected or rerouted to some predefined page. But the selection surely
will depend on how and where our website is hosted; we may not have access to
the IIS manager. In such cases, there are a variety of ways to take an
application offline through ASP.NET techniques. It can be <httpRuntime>
settings, App_Offline.htm or custom code. The newly added App_Offline.htm
feature in ASP.NET 2.0 is really a nice way for implementing an Under Construction
or Down for Maintenance message to all users while we are upgrading features,
fixing bugs, or making web design changes in a Website.
References
App_Offline.htm