Yes, the host header is basically your domain address.  But it is called the host header when it is used in the context of the Web server and the Web site definition, in our case within IIS.  You see, when you define a Web site the server can use different ways to identify what content you actually want and to match the requested site to the physical files on the server.  The parameters the server can use to identify the site are the TCP/IP address, the port number, and the host header.  Now, for public sites you want to use the standard port for the HTTP protocol, in other words port 80.  Then, because TCP/IP addresses are scarce and you can host many sites on a server with just one address, the only way left for IIS to distinguish what site you want is the host header.  So yes, the host header looks like www.yourdomain.com.

Now, let's keep it simple and say that you have one installation of DNN on the server, just one physical directory with the software.  Not only can IIS host multiple Web sites but so can DNN.  So, with DNN you can create a site for Domain1.com, another totally separate for Domain2.com, etc.  In this case, you have to add the host header for each one of the sites you create in DNN to the same Web site definition in IIS.  In other words, the one Web site will respond to the requests from all these domains.  In this case you would also have to add an alias for each site in DNN.  The situation of the alias in DNN is analogous to the host header in IIS, you have one service responding to requests from possibly different sites and it needs a way to distinguish which one it is.

By the way, if you want your server to respond to both "www.yourdomain.com" as well as just "yourdomain.com" you need to add both entries as a host header for the site.

Does this help?

Carlos

The answer is basically all the above.  By the way, I suppose the wizard you mention is the DNN wizard to create portals right?  Just wanted you to know that the wizard has been removed from newer versions of DNN, I would recommend you to create your new portals using the menu, which is the common way accross all versions.  You need to login as Host, go to the Host menu, then Portals, and on that screen you should see a link with the pencil icon that reads "Add New Portal".  When you click on it you get a form where you enter the info for the new portal including, you guess it, the portal alias.  The user name you enter there will be the Administrator of the new portal.

Once the new portal is created, go to the IIS admin and enter the host header for the one DNN Web site, it should match the alias you entered for the site.

By the way, if you have a dedicated, registered domain for the new portal this is easy.  If you do not, and still want to test the new portal you could use a subdomain of an already registered domain like, "newportal.olddomain.com".  In that case you enter that as both the alias and the host header.

Bonus tip, you can create any domain you like for testing and then use it for the alias and the host header.  The trick is to enter the new testing domain name in your Hosts file, located in C:\WINDOWS\system32\drivers\etc.  The Hosts file is a text file with no extension.  Just create a new line with the TCP/IP address of the server and the new domain name like:

127.0.0.1 fakedomain.com

Notice that you can do this trick even on your local machine using the address above, for a separate server use the real server address.  After you put this entry in the Host file you can use fakedomain.com in your browser to connect to the site.

Carlos