You could have those entries in hosts file you maintain checked before regular DNS lookup. nsookup can resolve using a hostsfile for reverse lookups.
Host name resolution generally uses the following sequence:
The client checks to see if the name queried is its own.
The client then searches a local Hosts file, a list of IP address and names stored on the local computer.
NOTE: The Hosts file location depends on the operating system:
Windows NT %Systemroot%\System32\Drivers\Etc
Where %Systemroot% is the folder in which Windows NT is installed
Domain Name System (DNS) servers are queried.
If the name is still not resolved, NetBIOS name resolution sequence is used as a backup. This order can be changed by configuring the NetBIOS node type of the client. (Note you have to enable this in Netflow DNS and NetBIOS Resolution under netflow setting if you want it to use NetBIOS for endpoint resolution)