What is happening is that you have configured your blog to use the domain as your main site. Therefore, when you try to access it via an unknown IP, it will redirect you to the
wp-signup.php page of the original blog, because it don't know which blog it is referring to.
This is the expected behavior of WordPress. It can also be customized, e.g. if you want to force a 404 when accessing the blog via an unknown domain/IP.
Note that, in your case, your administration page would be
/wp-admin from your main domain.