The Dynamic Address feature allows you to submit the credentials to the NAS with a dynamic IP address. In this model, the NAS will its own IP address during the captive portal redirect as a key value pair. Different vendors use different keys to inform the captive portal of its IP address. Here is a list of keys used per vendor.
Aruba / Dell / ALU: switchip
The Dynamic Address feature will use the value for these keys when the appropriate vendor is selected in the web login page. For example, the HTTPS POST for an Aruba vendor web login page will post to the value of switchip that the controller sent in the redirect URL.