Let me start off by saying I am very new to DNN, and web design in general, so bear with me.  I am not sure how to accomplish the following. 

I want to have a root site www.mydomain.com under that I would like to have three sub domains of voice.mydomain.com, network.mydomain.com, and store.mydomain.com (I am not sure if sub domains are the correct term or method to accomplish what I am trying to do).  Voice & network are two seperate department in our company and I am trying to have independent landing pages for each department & have a store that will sell items for both departments, however if a user goes to www.mydomain.com the menu system will include all options for all three sub-domains.  I want someone landing at voice.mydomain.com to be able to see the entire menu system so they can navigate to the network side of our business and vice versa.

What is the best/easiest way to accomplish this? 

Thank you in advance for you help.