Just as information, what happens in this process is that the user requests a site, such as www.sample.com. DNS resolves the www host in the sample.com domain to an IP address, say 192.168.1.10. The request is sent to that IP address. IIS answers on an IP address, it doesn't care what the URL is. If IIS is using host headers, IIS then sends the request to a site matching the host headers. If not, there is only a single site that responds to that IP and IIS sends the request to that site.
If that site is a DNN site, DNN looks at the request and reads the host/domain information, then checks to see if that is an alias of any of its portals. If that host/domain combination doesn't match any portal aliases, DNN loads the default portal. If that host/domain matches an alias, DNN loads the portal for which the alias matches.
So, If you have DNS for www.sample.org, www.sample.net and www.sample.com pointing to the same IP, IIS answers that IP and hands it to DNN. If DNN has three portals, and www.sample.com is the default, and the other two portals have aliases for bob.org and jim.net, then any request to www.sample.org will get answered by the www.sample.com portal since no alias matches www.sample.org. You must have correct aliases for the portals.
Likewise, if DNN is configured with www.sample.com, www.sample.org and www.sample.net as portals with those aliases, and someone browses to http://sample.org/, assuming a DNS entry exists for the blank host name (no www), then DNN will direct it to the www.sample.com domain since there is no alias for sample.org, only for www.sample.org.
Hope it all helps. Especially for anyone searching for this.
Jeff