grep -o '[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}' then how do i iterate the file names??

You don't really need iteration, just add more file names to the grep command line.

ohh well that works for me

i got the mistake i was making...i forgot to leave the space in one of the arguments

Exactly. The grep output changes so that each match is prefixed with the file name. There are options to turn that off if you don't need it.

if i then use the code to look up DNS then should i do it with nslookup host or dig?...i know dig is more useful which i read ....but if you could state the use of IFS then it will be great.

Recent versions of host can perform a reverse DNS lookup directly on the IP address. If you have an older version, or wish to use dig or some other tool, you have to convert the IP address into the format used internally for reverse DNS lookups. Here is an example.