cat ip.txt | cut -c 12-100
I think that 100 is enough, might not be the best approach but it works.
Edit:
Oh, and if you want to save the output to another file, try:
cat ip.txt | cut -c 12-100 | cat > ip2.txt
or
cat ip.txt | cut -c 12-100 | cat >> ip2.txt
Where ">" overrides specified file, and ">>" adds lines to existing file.