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A page extension of .asp indicates that the page contains code that can be interpreted into executable functions or operations and should be handled by passing the page content through the asp interpreter (ASP.DLL). However it is possible to add other extensions to the mappings and allow the pages to be parsed as executable scripts.

Changing the mappings can be setup as Global where all the sites configured with the same mappings, however this will put a slight extra load on the server as more files have to be passed through an extra stage but unless there are a large number of virtual hosts on the server is unlikely to be an issue.
Mappings can be set for each individual site configured on the server. This still increases the processing load but obviously not as much as a server-wide change would.

To start the process open the IIS snap-in from;
Start -> Programs -> Administrative Tools -> Internet Services Manager
or
Start -> Settings -> Control Panel -> Administrative Tools -> Internet Services Manager.

For the Global configuration, Highlight the server name and from the menu select Action -> properties.
For individual site(s), Select the required site name and from the menu select Action -> properties.