Hello Kai,
have you enabled the check-box for DNS lookup in
ServerConfiguration>ANI Server Admin>Setup>DiscoverySettings
Enable it, apply the changes and run the discovery immediately. After it has finished the devices should be discoverd by name. You can check it under
ServerConfiguration>Diagnostic>DiscoveryMetrics
Click on the number of discoverd devices; there you get a list with IP addresses and hostnames if available.
regards,
Martin