Every domain name, which is a part of the DNS system, has several DNS settings, also known as DNS records. In order for these DNS records to be kept in order, the DNS zone was created. It contains all the records and settings (e.g SOA) of a domain name. Every domain name has one or more DNS zones, delegated to a specific DNS management host/service. They could use different configuration - one Master DNS zone at ClouDNS or one Master DNS zone at ClouDNS and one Slave/Backup DNS zone at a different DNS provider or vice versa, etc.
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