Ico files are almost obsolete, but only almost. We recommend that you still have a custom favicon.ico in your DNN root folder for various reasons. To create it, proceed as follows:

1. Create a square PNG file (we recommend 500x500 pixels and the name favicon500.png) which should be your favicon and compress it as best as possible
2. To generate a favicon.ico with multiple sizes, upload it to the favicon generator. Since you only want the favicon, you can ignore all the optional settings. 
   
3. Download the resulting zip and just take out the favicon.ico from there - you won't need the other files