code: public static String defaultOrigin(HttpServerExchange exchange) {
          String host = NetworkUtils.formatPossibleIpv6Address(exchange.getHostName());
          String protocol = exchange.getRequestScheme();
          int port = exchange.getHostPort();
          //This browser set header should not need IPv6 escaping
          StringBuilder allowedOrigin = new StringBuilder(256);
          allowedOrigin.append(protocol).append("://").append(host);
          if (!isDefaultPort(port, protocol)) {
              allowedOrigin.append(':').append(port);
          }
          return allowedOrigin.toString();
      }
docs: Determine the default origin, to allow for local access.
  @param exchange the current HttpExchange.
  @return the default origin (aka current server).