public static String toInAddrArpa(String ip){
		String[] ipn = ip.split("\\.");
		try{
			if(ipn.length==4){
				for(int i=0; i<4; i++){
					int num = Integer.parseInt(ipn[i]);
					if(num<0 || num>255) throw new NumberFormatException();
				}
				return ipn[3]+"."+ipn[2]+"."+ipn[1]+"."+ipn[0]+".in-addr.arpa";
			}
		}
		catch(NumberFormatException e){};
		return null;
    }