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What is a primary DNS zone?
The primary zone — historically the master — is the writable copy of a zone. Every change starts here.
Records are added, changed and deleted on the primary. It raises the serial and then tells the secondaries that something happened.
Only the primary is writable. That is deliberate: one place to change, one place to audit.
At CTPFDNS the primary stands in Frankfurt. It notifies Amsterdam, London and Los Angeles the moment a zone changes.
A zone can be primary here even when the domain is registered elsewhere — the registrar only needs to delegate to our nameservers.