Updated June 13, 2016
This article provides information when you get the following prompt:
Hostnames Not Resolved for some IP Addresses that are "Used"
All versions of IPAM including version 4.3.1
IP Addresses that are marked "Used" by Neighbor Scanning do not have their hostnames resolved. It is only when IP Addresses are marked "Used" by standard ICMP scan that have the hostnames resolved.
This is by design.
A Feature Request has been logged with development to add hostname lookup for IP Addresses marked as "used" by Neighbor scanning in the future.