Problem running Nmap

I m new to Nmap and to linux rather
have the following problem running Nmap scanner

nmap -O x.x.x.x
when i run the above command i get the following (infact for all the
commands)

Starting Nmap 4.20 ( http://insecure.org ) at 2008-07-06 11:46 IST
mass_dns: warning: Unable to open /etc/resolv.conf. Try using --system-dns
or specify valid servers with --dns-servers
mass_dns: warning: Unable to determine any DNS servers. Reverse DNS is
disabled. Try using --system-dns or specify valid servers with --dns_servers
Note: Host seems down. If it is really up, but blocking our ping probes, try
-P0
Nmap finished: 1 IP address (0 hosts up) scanned in 0.171 seconds

whats this message is all about how do i resolve it .

Regards
Neminath

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