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1. Scope/Audience

1. When considering a recursive DNS resolver:
   1. Is the resolver service public or private ?
   2. Is the resolver service open or closed ?
2. Clarification:
   1. Public: can be reached over the open internet (public IP address, not restricted)
   2. Private: cannot be reached over the open internet (private IP address, or ACL restrictions, or a combination)
   3. Open: reachable by, and responds to queries from any client
   4. Closed: requires authentication of some sort to be used
      1. IP address, TSIG, TLS cert (DoT)

2. In practice, the following services are found on the internet:

1. Private Resolvers - Found in corporate / restricted networks, not publicly accessible.

2. Shared Private Resolvers - ISPs or similar hosting service providers

3. Public Resolver Operators - Commercial DNS filtering / scrubbing service.