how to find host name from IP with out login to the host

i need to find the host name of a UNIX host whose IP is known with out login to that UNIX host

From stackoverflow

• Use nslookup

  nslookup 208.77.188.166 
  ...
  Non-authoritative answer:
  166.188.77.208.in-addr.arpa     name = www.example.com.
  

• You can use traceroute command as well.

  http://linux.die.net/man/8/traceroute

  just use the traceroute it will show you the routing path with host names (IPs resolved)

• You can do a reverse DNS lookup with host, too. Just give it the IP address as an argument:

  $ host 192.168.0.10
  server10 has address 192.168.0.10
  

• The other answers here are correct - use reverse DNS lookups. If you want to do it via a scripting language (Python, Perl) you could use the gethostbyaddr API.