Why some devices hostname shown with IP Address during agentless discovery?

SapphireIMS performs DNS reverse lookup based on IP and if it unable to resolve to host name, we simply store IP Address in the field host name hence this behavior.
The possible reasons are –
a. The DNS server IP Address is not properly configured in SapphireIMS server
b. The DNS Server is not reachable
c. The DNS Server is down during discovery
d. The DNS Server does not exist
e. The DNS configuration may not be correct to do reverse lookup