Magento 2 native doesn't have feature to show customer IP who is online. There is other way, you have to create one module for display ip in back-end online customer grid. In that module create one di.xml file and use customer login event for get and save customer ip in 'customer_visitor' table. Create one column in customer online grid at back-end and display this ip from 'customer_visitor' table. You can get IP via below code.
$om = \Magento\Framework\App\ObjectManager::getInstance();
$obj = $om->get('Magento\Framework\HTTP\PhpEnvironment\RemoteAddress');
$ip = $obj->getRemoteAddress();
I hope this will help you.