If you try to installing an application on Server 2012 that requires .NET Framework 3.5 and got an error message indicating to enable .Net feature.
Bellow is one of the solution to install .NET Framework on Server 2012.
1. Insert the Windows Server 2012 DVD into your server
2. Run this command in the command line as administrator dism /online /enable-feature /featurename:NETFX3 /all /Source:d:\sources\sxs /LimitAccess – assuming your server’s CDROM drive is D.
3. After successfully installing the .Net Framework, you should be able to install any other software that require .Net 3.5