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function wp_privacy_anonymize_ip( $ip_addr, $ipv6_fallback = false ) {
	if ( empty( $ip_addr ) ) {
		return '0.0.0.0';
	}

	// Detect what kind of IP address this is.
	$ip_prefix = '';
	$is_ipv6   = substr_count( $ip_addr, ':' ) > 1;
	$is_ipv4   = ( 3 === substr_count( $ip_addr, '.' ) );

	if ( $is_ipv6 && $is_ipv4 ) {
		// IPv6 compatibility mode, temporarily strip the IPv6 part, and treat it like IPv4.
		$ip_prefix = '::ffff:';
		$ip_addr   = preg_replace( '/^\[?[0-9a-f:]*:/i', '', $ip_addr );
		$ip_addr   = str_replace( ']', '', $ip_addr );
		$is_ipv6   = false;
	}

	if ( $is_ipv6 ) {
		// IPv6 addresses will always be enclosed in [] if there's a port.
		$left_bracket  = strpos( $ip_addr, '[' );
		$right_bracket = strpos( $ip_addr, ']' );
		$percent       = strpos( $ip_addr, '%' );
		$netmask       = 'ffff:ffff:ffff:ffff:0000:0000:0000:0000';

		// Strip the port (and [] from IPv6 addresses), if they exist.
		if ( false !== $left_bracket && false !== $right_bracket ) {
			$ip_addr = substr( $ip_addr, $left_bracket + 1, $right_bracket - $left_bracket - 1 );
		} elseif ( false !== $left_bracket || false !== $right_bracket ) {
			// The IP has one bracket, but not both, so it's malformed.
			return '::';
		}

		// Strip the reachability scope.
		if ( false !== $percent ) {
			$ip_addr = substr( $ip_addr, 0, $percent );
		}

		// No invalid characters should be left.
		if ( preg_match( '/[^0-9a-f:]/i', $ip_addr ) ) {
			return '::';
		}

		// Partially anonymize the IP by reducing it to the corresponding network ID.
		if ( function_exists( 'inet_pton' ) && function_exists( 'inet_ntop' ) ) {
			$ip_addr = inet_ntop( inet_pton( $ip_addr ) & inet_pton( $netmask ) );
			if ( false === $ip_addr ) {
				return '::';
			}
		} elseif ( ! $ipv6_fallback ) {
			return '::';
		}
	} elseif ( $is_ipv4 ) {
		// Strip any port and partially anonymize the IP.
		$last_octet_position = strrpos( $ip_addr, '.' );
		$ip_addr             = substr( $ip_addr, 0, $last_octet_position ) . '.0';
	} else {
		return '0.0.0.0';
	}

	// Restore the IPv6 prefix to compatibility mode addresses.
	return $ip_prefix . $ip_addr;
}