my $x = '255.0.0.0';
print "$x is a valid mask!\n" if validateMask($x);

sub validateMask {
  return                                # return undef if it's not complete
    unless $_[0] =~/^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/;
  $_[0] = unpack (
          "B32",
          pack( 
               "C4", 
               split ( /\./, $_[0] )
              )
         );

  return                                # return undef if we find an invalid
    if $_[0] =~ /01/;                   # mask ( a zero followed by a one )
  return 1;                             # return 1 if everything is okay
}