How to do it...

1. Open Server Manager on Windows Server 2016, and click on the highlighted Add roles and features option.

1. Click on Next until you get the Select features wizard.

1. Click on the Features tab and click on the checkbox for .NET Framework 3.5 Features. It will also install the supported 2.0 framework. Move to the next screen:

1. There is a warning displayed before installation: Do you need to specify an alternate source path? One or more installation selection are missing sources files on the destinati... (it goes on for a bit; hover on it to read the full warning).
2. We have to provide the installation media's sources\sxs folder path. Click on Specify an alternate source path. You'll get this screen:

1. Here, I have the Windows Server 2016 media in the D: drive. This is the media path in my case, which I have downloaded, but in your case, it can be different or be an actual optical drive. There is a folder called sources and a subfolder, sxs. Inside this folder, the installation file is available:

1. Now you know where the source folder is. Go to the confirmation screen and click on Install. The next screenshot shows the installation progress on WIN2016IIS:

1. Click on Close when the installation is complete; you now have framework 3.5 available on your server.
2. Now you have to verify that framework 3.5 has been installed. It should be available in the Features wizard.
3. Open Server Manager and click on Add roles and features.

1. Keep clicking on Next until you get the Select Features wizard. You will see the .NET framework 3.5 checkbox checked, and the box will be gray, meaning it's disabled:  

1. We've confirmed that .NET v3.5 has been installed on our server.