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DNS

There are three classes of DNS servers:

Root DNS servers
1. There are more than 2000 root servers world wide. They are copies of 13 different root servers
Top Level Domain DNS servers
Authoritative DNS servers

Finding authoritative DNS servers:

dig NS apple.com +short  # the NS record identifies which servers are supposed to be authoritative;
                         # querying those servers directly and checking for aa confirms they actually are