#!bin/perl
#
#   mask-cvt.pl -- a script to convert between the various
#                  methods of masking IP addresses
#
sub usage(  ) {
   print "mask-cvt [nwb] [nwb] {nnn.nnn.nnn.nnn|bbb}\n";
   print "     where the first argument, [nwba], specifies the input \n";
   print "           format as one of netmask, wildcard or number of \n";
   print "           bits and the second argument, [nwb], specifies \n";
   print "           the output format\n";
   
   exit(  );
}
   
if($#ARGV != 2) { usage(  ); }
   
# get the input format style
$_ = @ARGV[0];
   
if(/[nN]/) {
      # incoming format netmask, what's the outgoing
      $_ = @ARGV[1];
      if(/[nN]/) {
         # no conversion
         $output = @ARGV[2];
         } elsif (/[wW]/) {
            # out is wildcard
            $output = do_subtract(@ARGV[2]);
         } elsif (/[bB]/) {
            # out is wildcard
            $output = do_bits(@ARGV[2]);
         } else {
            usage(  );
         }
  } elsif (/[wW]/) {
      # incoming format wildcard, what's the outgoing
      $_ = @ARGV[1];
      if(/[wW]/) {
         # no conversion
         $output = @ARGV[2];
         } elsif (/[nN]/) {
            # out is wildcard
            $output = do_subtract(@ARGV[2]);
         } elsif (/[bB]/) {
            # out is wildcard
            $output = do_bits(do_subtract(@ARGV[2]));
         } else {
            usage(  );
         }
  } elsif (/[bB]/) {
      # remove any leading "/" in the bit count
      $_ = @ARGV[2];
      s/[-\/]//;
      $bits = $_;
      
      # incoming format is bit count, what's the outgoing
      $_ = @ARGV[1];
      if(/[bB]/) {
         # no conversion
         $output = @ARGV[2];
         print "no conversion\n";
         } elsif (/[nN]/) {
             # out is netmask
             $output = cvt_bits_mask($bits);
         } elsif (/[wW]/) {
            # out is wildcard
            $output = do_subtract(cvt_bits_mask($bits));
         } else {
           usage(  );
         }
  } else {
     usage(  );
}
   
print "$output\n";
   
sub do_subtract(  ) {
  local($ip) = @_;
   
  # break up the bytes of the incoming IP address
  $_ = $ip;
  ($a, $b, $c, $d) = split(/\./);
   
  if ($a > 255 || $b > 255 || $c > 255 || $d > 255 || /[^0-9.]/) {
     print "invalid input mask or wildcard\n";
     exit(  );
  }
   
  $a = 255 - $a;
  $b = 255 - $b;
  $c = 255 - $c;
  $d = 255 - $d;
   
  return ($a . "." . $b . "." . $c . "." . $d);
}
   
sub do_bits(  ) {
  local($ip) = @_;
  
  # break up the bytes of the incoming IP address
  $_ = $ip;
  @ip_bytes = split(/\./);
   
  if ($ip_bytes[0] > 255 || $ip_bytes[1] > 255 || $ip_bytes[2] > 255 
       || $ip_bytes[3] > 255 || /[^0-9.]/ || $#ip_bytes != 3) {
     print "invalid input mask or wildcard\n";
     exit(  );
  }
   
  $bits = 0;
  for ($i=0; $i < 4 ; $i++) {
     if ($ip_bytes[$i] > 0 && $bits < 8*$i) {
        print "invalid mask for bit count format\n";
        exit(  );
     }
     if ($ip_bytes[$i] =  = 255 ) { $bits += 8;
     } elsif ($ip_bytes[$i] =  = 254 ) { $bits += 7; 
     } elsif ($ip_bytes[$i] =  = 252 ) { $bits += 6; 
     } elsif ($ip_bytes[$i] =  = 248 ) { $bits += 5; 
     } elsif ($ip_bytes[$i] =  = 240 ) { $bits += 4; 
     } elsif ($ip_bytes[$i] =  = 224 ) { $bits += 3; 
     } elsif ($ip_bytes[$i] =  = 192 ) { $bits += 2; 
     } elsif ($ip_bytes[$i] =  = 128 ) { $bits += 1; 
     } elsif ($ip_bytes[$i] != 0 ) {
        print "invalid mask for bit count format\n";
        exit(  );
     }
   }
   return("/" . $bits);
}
sub cvt_bits_mask(  ) {
   local($bits) = @_;
   
   if ($bits <= 8 ) {
     $a = bits_to_dec($bits);
     $b=$c=$d=0;
   } else {
     $a=255;
     if ($bits <= 16 ) {
        $b = bits_to_dec($bits-8);
        $c=$d=0; 
     } else {
        $b=255;
        if ($bits <= 24 ) {
           $c = bits_to_dec($bits-16);
           $d=0;
        } else {
           $c=255;
           if ($bits <= 32 ) {
              $d = bits_to_dec($bits-24);
           } else {
              print "invalid bit count\n";
              exit(  );
           }
        }
     }
   }
   return ($a . "." . $b . "." . $c . "." . $d);
}
   
sub bits_to_dec(  ) {
   local($bits) = @_;
   
   if($bits =  = 0 ) { return 0; }
   if($bits =  = 1 ) { return 128; }
   if($bits =  = 2 ) { return 192; }
   if($bits =  = 3 ) { return 224; }
   if($bits =  = 4 ) { return 240; }
   if($bits =  = 5 ) { return 248; }
   if($bits =  = 6 ) { return 252; }
   if($bits =  = 7 ) { return 254; }
   if($bits =  = 8 ) { return 255; }
}