Least privilege means giving exactly the access needed and nothing more. In networking, this applies at multiple levels:
User access. Users reach only applications required for their job function. The marketing team doesn't need access to development servers.
Service accounts. Applications connect only to their required dependencies. A web server reaches its database, not every database in the data center.
Network protocols. If a connection needs HTTPS, block everything else between those endpoints. Don't allow unnecessary protocols "just in case."