Question: How to create a private knowledge base for internal use only?

Our company has a lot of internal documentation and process guides that we want to store in a knowledge base. However, this information is sensitive and should only be accessible to our employees. We don't want this content to be public or indexed by search engines. What is the best way to set up a completely private, internal-only knowledge base? Do we need a separate instance, or can we restrict access to a specific section of our existing Help Center? I've looked at the user segment options, but I'm not sure if that's secure enough. I need to ensure that only logged-in agents and admins can view or search for these articles. Any guidance on the correct settings and permissions would be greatly appreciated.