Cara menampilkan user yg sedang online

Perimisi gan, ane mau bertanya. bagaimana cara menampilkan siapa saja user yang sedang online.

user_table

id name username password alamat   kontak
1  anton  anton    ****   bogor   6281****
2  lala   lala     ****   depok   6281****

Controllernya

public function login(){
        $data = array(
         'judul' => 'Masuk'
        );
        $this->form_validation->set_rules('username', 'Username', 'trim|required');
        $this->form_validation->set_rules('password', 'Password', 'trim|required');

        if($this->form_validation->run() === FALSE){
            $this->load->view('global/header');
            $this->load->view('administrators/login', $data);
            $this->load->view('global/footer');
        }else{
            $username = $this->input->post('username');
            $password = md5($this->input->post('password'));
            $user_id = $this->administrator_model->login($username, $password);
            if($user_id->num_rows() > 0){
                $data  = $user_id->row_array();
                $kontak = $data['kontak'];
                $alamat = $data['alamat'];
                //session
                $user_data = array(
                    'user_id' => $user_id,
                    'kontak' => $kontak,
                    'alamat' => $alamat,
                    'logged_in' => true
                );
                // print_r($user_data);
                // die();
                $this->session->set_userdata($user_data);
            }else{
                $this->session->set_flashdata('login_failed', 'Username atau password anda salah');
                redirect('administrators/login');
            }
        }
    }

Modelnya

public function login($username,$password){
   $this->db->where('username',$username);
   $this->db->where('password',$password);
   $result = $this->db->get('administrators',1);
   return $result;
    }

Kira2 apa yang mesti ane tambah agar ane tahu siapa aja user yang sedang online. Terima kasih sebelumnya...