.NET Framework 3.5 install on a Windows 2012 machine

For some applications it's required to have .NET Framework 3.5 installed on your Windows 2012 machine. Some of these applications include SQL 2014 and CRM 2015.

The ususal option is to select the ".NET Framework 3.5 Features" in the "Add Roles and Features Wizard" of the windows server manager. However, in some scenario's this leads to the following error message:

Installation of one or more roles, role services, or features failed.
The source files could not be found. Try installing the roles, role services, or features again in a new Add Roles and Features Wizard session, and on the Confirmation page of the wizard, click "Specify an alternate source path" to specify a valid location of the source files that are required for the installation. The location must be accessible by the computer account of the destination server.

Now if you have the windows 2012 disc at hand, you can follow the next statement.
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:d:\sources\sxs
If you don't have the windows 2012 disc at hand, you can force the installer to grab .NET Framework 3.5 from the Windows Update. You can do that using the following statement in the command prompt:
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All

For more details see Technet: https://technet.microsoft.com/en-us/library/hh831809.aspx#BKMK_dism