A NameServer that is authoritative for a particular DomainName, which means that anything it doesn't know about that domain, doesn't exist. Authoritative name servers are listed in NS lines in the initial part of the zone file.
Standard DNS policy requires that every domain have at least two authoritative name servers. It is a good idea to have these machines in separate physical locations, to guard against accidents knocking both/all of them out at the same time.
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