The aspnet.config file is a little known config file which is supported by ASP.NET 2.0 and greater. Generally it lives in the root of the framework folder, for example: C:\Windows\Microsoft.NET\Framework64\v4.0.30319\aspnet.config C:\Windows\Microsoft
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Read the complete post at http://blogs.iis.net/owscott/archive/2011/12/01/setting-an-aspnet-config-file-per-application-pool.aspx