Default.aspx

ASP.NET web page Reference

An .aspx file is a web page for ASP.NET Web Forms. The file mixes HTML markup with server controls and code references. The IIS web server compiles the page and sends the resulting HTML to the browser. The code-behind file with the page logic has the extension .aspx.cs or .aspx.vb. Newer ASP.NET Core projects use Razor pages in its place.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.