Server 2012 - Failed to Turn On .Net 3.5 Fx Features ~ virtually n00b

I am using Windows Server 2012 for my XenDesktop 7 lab test. During the installation of SQL server, it prompted an error, asking for .Net 3.5 Framework to be turned on. I tried to enable it, but failed. Thinking and thinking, until I realized what was the issue

Issues :

Received below error while trying to enabling .Net 3.5 Framework feature in Windows Server 2012.

Error :

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.

Troubleshooting

It turns out that to enable the feature, the source is needed. In this case, it will grab the source from internet.

Resolution :

Mount the Windows Server 2012 CD / ISO to the drive

Open command prompt as administrator, and launch this command :
dism.exe /online /enable-feature /all /featurename:NetFX3 /Source:D:\sources\sxs /LimitAccess