Allowed Domains are domains to which the end users can browse while the end-system is being assessed, the end-system is quarantined, or when the end-system is not registering on the network. The NAC will proxy the HTTP to the allowed domains as long as it is configured with a DNS server. It will answer these request as if it was the server.To configure:
In NAC Manager
NAC Manager > Tools > Management and Configuration > NAC Configuration > Portal > Network Settings > click Allowed Web Sites: change > click Allowed Domains tab
In Extreme Management Center
Note that a ! in front of the website will block access to the website from the nac in proxying these captive portal packets.
This is often beneficial when a certain provider is not working within an existing captive portal or pop up. Adding or removing these may be helpful.