How can I convert IPV6 address to IPV4 address?

Here is the code you are looking for in javascript. Well you know you can't convert all of the ipv6 addresses

<script>
function parseIp6(str)
{
  //init
  var ar=new Array;
  for(var i=0;i<8;i++)ar[i]=0;
  //check for trivial IPs
  if(str=="::")return ar;
  //parse
  var sar=str.split(':');
  var slen=sar.length;
  if(slen>8)slen=8;
  var j=0;
  for(var i=0;i<slen;i++){
    //this is a "::", switch to end-run mode
    if(i && sar[i]==""){j=9-slen+i;continue;}
    ar[j]=parseInt("0x0"+sar[i]);
    j++;
  }

  return ar;
}
function ipcnvfrom6(ip6)
{
  var ip6=parseIp6(ip6);
  var ip4=(ip6[6]>>8)+"."+(ip6[6]&0xff)+"."+(ip6[7]>>8)+"."+(ip6[7]&0xff);
  return ip4;
}
alert(ipcnvfrom6("::C0A8:4A07"));
</script>

The IPAddress Java library can accomplish what you are describing here.

IPv6 addresses are 16 bytes. Using that library, if you are starting with a 16-byte array you can construct the IPv6 address object:

IPv6Address addr = new IPv6Address(bytes); //bytes is byte[16]

From there you can check if the address is IPv4 mapped, IPv4 compatible, IPv4 translated, and so on (there are many possible ways IPv6 represents IPv4 addresses). In most cases, if an IPv6 address represents an IPv4 address, the ipv4 address is in the lower 4 bytes, and so you can get the derived IPv4 address as follows. Afterwards, you can convert back to bytes, which will be just 4 bytes for IPv4.

    if(addr.isIPv4Compatible() || addr.isIPv4Mapped()) {
        IPv4Address derivedIpv4Address = addr.getEmbeddedIPv4Address();
        byte ipv4Bytes[] = derivedIpv4Address.getBytes();
        ...
     }

The javadoc is available at the link.

While there are IPv6 equivalents for the IPv4 address range, you can't convert all IPv6 addresses to IPv4 - there are more IPv6 addresses than there are IPv4 addresses.

The only sane way around this issue is to update your application to be able to understand and store IPv6 addresses.

There isn't a 1-1 correspondence between IPv4 and IPv6 addresses (nor between IP addresses and devices), so what you're asking for generally isn't possible.

There is a particular range of IPv6 addresses that actually represent the IPv4 address space, but general IPv6 addresses will not be from this range.