DNS query – NSlookup

PDC is windows 2003 – server name is A2
SDC is win 2003 – name is A1

when i fire NSLOOKUP command….
e.g.
c:>nslookup 192.168.0.XX (XX is PDC)…. this command shows follwoing output

*** can’t find servername for address 192.168.0.YY (this is address of SDC):non-exisisting domain
server: unknown
address:192.168.0.YY

*** Unknown can’t find 192.168.0.XX: non exisisting domain

what does this message mean?

can anybody explain this? ):

thaks.