http://support.microsoft.com/default.aspx?scid=kb;en-us;171789
SYMPTOMS
When you run Nslookup from a command prompt on your Microsoft Windows NT 4.0
Domain Name Service (DNS) server, you receive the following error message:
**** Can't find server name for address x.x.x.x: Non-existent domain
**** Default server is not available
Default Server: UnKnown
Address: x.x.x.x
Where x.x.x.x is the IP address of your DNS server.
NOTE: This problem may occur with other applications as well.
CAUSE
There is a missing or mis-configured PTR record for your DNS server in your
reverse lookup zone. The DNS Manager does not automatically create a PTR
record for your DNS server, even though it may automatically create an "A"
record in your forward zone.
RESOLUTION
To resolve this problem, perform the following steps:
1.. From DNS Manager, open your forward zone.
2.. Right-click on the "A" record for your DNS server, and then click
Delete Record.
3.. Click Yes to confirm the deletion of the "A" record.
4.. Right-click on the forward zone, and then click New Host.
5.. Type the host name of your DNS server and the IP address.
6.. Select the Create Associated PTR Record check box.
7.. Click Add Host, and then click Done.
If you receive an error when you click Add Host, then you may be having a
problem with your reverse lookup zone.
For additional information, please see the following articles in the
Microsoft Knowledge Base:
166753 Microsoft DNS Server Reverse Lookup Error Adding Host Record
162842 DNS with "WINS Reverse Lookup" Fails with Multiple IP Addresses
Noriega
A+, MCP, MCSA