This is an excerpt from the official documentation in the section "Reverse DNS Considerations" Form : https://docs.microsoft.com/en-us/azure/virtual-network/virtual-networks-name-resolution-for-vms-and-role-instances#dns-client-configuration [..." - All PTR queries for IP addresses of virtual machines will return FQDNs of form [vmname].internal.cloudapp.net - Forward lookup on FQDNs of form [vmname].internal.cloudapp.net will resolve to IP address assigned to the virtual machine. - If the virtual network is linked to an Azure DNS private zones as a registration virtual network, the reverse DNS queries will return two records. One record will be of the form [vmname].[privatednszonename] and the other will be of the form [vmname].internal.cloudapp.net "...]
https://docs.microsoft.com/en-us/azure/virtual-network/virtual-networks-name-resolution-for-vms-and-role-instances