Geometric networks are part of core ArcGIS.
Hello, Jay,
I have a full license ArcMap 10.5.1 I don't see geometric networks.
Geometric network support is available in core ArcGIS. How are you trying to see them? Do you have some data with geometric networks? Can you use Catalog to view it? Can you add the data to map? There is a toolbar titled "Utility Network Analyst" that you can bring up that will let you work with analysis on these geometric networks.
If you have installed the tutorial data, try going through this tutorial:
Exercise 5: Build a geometric network—ArcGIS Help | ArcGIS Desktop
Jay Sandhu