The built in web server in Visual Studio (IIS Express for VS2010 and newer) is a convenient built in dev environment. It's much easier to use than full IIS and is the default for new web projects. While developing in a local copy of IIS may be needed for some customers, for those who want a simpler (zero configuration) development server, it's often quite suitable

The one big problem it's had has been that since Visual Studio itself is a 32 bit process, IISExpress was defaulted to running 32 bit. This has made it difficult to develop and test in 64 bit. For a long time, the solution was to use a local IIS instance.

However, it is possible to toggle IIS Express's "bitness" to 64 or 32

VS2013 and newer

For Visual Studio 2013 and newer, this is a fully supported feature, built in and just needs a quick dive into the options menu

Go to Tools -> Options on the main menu
Then in the Options dialog, expand "Projects and Solutions"
Now, go to the Web Projects tab
Check (or uncheck) the "use the 64 bit version of IIS Express for Web Sites and Projects" checkbox
Click OK

VS2012

For Visual Studio 2012,it requires a registry hack

It is STRONGLY recommended to use VS2013 or later if you need IIS x64 support and not to use the registry hack as it's not 100% reliable

Run Regedit
Navigate to HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\11.0\WebProjects
if the RegDword Use64BitIISExpress exists, set its value to 1 to enable 64 bit (0 to use 32 bit)
if the RegDword Use64BitIISExpress does not exist, create it and set to 1 to enable 64 bit (0 to use 32 bit)

VS2010 and Older

The built in web server does not support x64 in VS2010 and older - if you must use one of these IDEs you'll need to use a local copy of IIS to develop/test in x64

For more information, please see:

INFO: Bitness Roundup Whitepaper: x86, x64, AnyCPU

Original Article:
Q10450 - HOWTO: IISExpress in 64 bit / 32 bit