Reason: The host name will be set in the Browser Cookies for session tracking. As both Admin server and Managed Server are running in the same server, the host name will be same for Admin Server and the Managed Server and you cannot navigate to the Admin and the Managed Server at the same time.
For example, say I am running Admin and the Managed Server in the same server AppSer1. The IP Address of the server AppSer1 is 192.168.112.96.
And the fully qualified domain name of the server AppSer1 is AppSer1.company.com.
You can connect one server with the host name and another server with IP address. Also you can try one with host name and another server with domain name. If you want to connect to Applications Manager from the localhost, you can also connect using localhost.
Possible options are,
Options 4 & 5 can be used, if you want to connect from the localhost, where Applications Manager is running.
Also you can have two host names for the same server and can access them independently.