Changes since SOA version
IXFR is a zone transfer request that returns changes since a specific SOA version. Enter the SOA version to request changes since then.
Not an apex domain
The domain 212-51-137-231.fiber7.init7.net is not the root of its zone.
Instead, try zone transfer on fiber7.init7.net .
Instead, try zone transfer on fiber7.init7.net .