To facilitate this interaction, servers running Windows Server® 2008 and DHCP and clients running DHCP can register with DNS, allowing cooperation between the two.
When DHCP changes IP address information, corresponding DNS updates synchronize name-to-address associations for the computer.
Use Azure DNS to host your Domain Name System (DNS) domains in Azure.
Manage your DNS records using the same credentials, and billing and support contract, as your other Azure services.
To make myself understood I'll give you this example: A computer connects to VPN, it gets an IP from another subnet, the DNS is updated accordingly.
The computer gets disconnected from the VPN and get connected straight to the network but the DNS doesn't update the hostname with the new IP.
DNS servers provide domain name resolution for network resources.