Hi All,
A new blog article, Three Methods for Clipping a Network Dataset in ArcGIS Desktop, was just published. I see this question come to Esri Support fairly frequently, especially with the rise of Navigator and the ability to use custom data in it (particularly Street Map Premium Custom Roads). So now we have the answer.
If you've ever wondered how to clip your network dataset, check out the article!
https://community.esri.com/groups/technical-support/blog/2017/06/23/three-methods-for-clipping-a-net...
-Rachel