grep(1) -oP '\d+\.\d+\.\d+\.\d+(\/.\d+|)' file_with_ips
print lines matching a pattern
-o, --only-matching
       Print  only  the  matched  (non-empty) parts of a matching line, with each such part on a separate
       output line.
-P, --perl-regexp
       Interpret PATTERN as a Perl regular expression (PCRE, see below).  This is highly experimental and
       grep -P may warn of unimplemented features.
grep searches the named input FILEs (or standard input if no files are named, or if a single hyphen-minus
(-) is given as file name) for lines containing a match to the given PATTERN.  By  default,  grep  prints
the matching lines.
source manpages: grep