base_url config from 2.x breaking change than 2.1.x

I'm using old 2.1.x version, config file set as below

PHP Code:

$config['base_url']    = ''; 


 In lastest 2.x version, when i set it is empty, it only use ip address in my url instead of domain name with older version
Along time i dont work with PHP and how we need set this config value instead of default
king