4rd-Free.js

<script src="ipaddr.min.js">
/* https://github.com/whitequark/ipaddr.js/ */
</script>

<script>

/*
from: https://lafibre.info/free-10g-epon/zones-de-test-zmd/msg312896/#msg312896
*/

var pfx4 = ipaddr.parse("91.0.0.0");
var pfx6 = ipaddr.parse("2a01:e00::");

function is_in_Free_4rd_dom(ip6)
{
  return ip6.match(pfx6,24); // TODO: /24 is a guess, get the real Free/Iliad 4rd prefix
}

function ip6_to_ip4(ip6) {
  var ip4 = ipaddr.fromByteArray([
    pfx4.octets[0],
    ((ip6.parts[1] & 0xF) << 4) | ((ip6.parts[2] >> 8) & 0x3),
    ((ip6.parts[2] & 0x3F) <<2) | (ip6.parts[3] >> 14),
    (ip6.parts[3] >> 6) & 0xFF
    ]
  );

  return ip4;
}

function ip6_to_ip4_port_range(ip6) {
   //(ip6 << 10) & 0xc000
   return ((ip6.parts[3]>>8) & 0x3);
}

function port_range_to_range(pr) {
  switch(pr)
  {
     case 0: return {from:0    , to:16383};
     case 1: return {from:16384, to:32767};
     case 2: return {from:32768, to:49151};
     case 3: return {from:49152, to:65535};
   }
}

/*
function ip4_to_ip6(ip4,port_range)
{
    //TODO
}
*/
</script>