$config['sess_cookie_name'] = 'cb_session';
$config['sess_expiration']  = 0;
$config['sess_expire_on_close'] = FALSE;
$config['sess_encrypt_cookie']  = TRUE;
$config['sess_use_database']    = TRUE;
$config['sess_table_name']  = 'cb_sessions';
$config['sess_match_ip']    = FALSE;
$config['sess_match_useragent'] = TRUE;
$config['sess_time_to_update']  = 300;

$config['cookie_prefix']    = "";
$config['cookie_domain']    = ".localhost";
$config['cookie_path']      = "/";
$config['cookie_secure']    = FALSE;

public function login($data){
//...Проверка введенных данных...
        $this->session->set_userdata($authdata);

CREATE TABLE IF NOT EXISTS `ci_sessions`
(

 `session_id` varchar(40) NOT NULL DEFAULT '0',

 `ip_address` varchar(16) NOT NULL DEFAULT '0',

 `user_agent` varchar(120) NOT NULL,

 `last_activity` int(10) unsigned NOT NULL DEFAULT '0',

 `user_data` text NOT NULL

) ENGINE=InnoDB

$config['base_url'] = 'http://sваш домен';

$config['cookie_prefix']    = "ваш префикс_";
$config['cookie_domain']    = "ваш домен";
$config['cookie_path']      = "/";
$config['cookie_secure']    = FALSE;