This is actually WAY simpler than most folks want to try and make it. I'll try to stick with your example, you'll be pleasantly surprised by how easy this is.
Setting the stage:
You have a machine which houses IIS. That machine recieves HTTP requests from some list of IP addresses ( which includes 123.45.678.90, 123.45.678.91 ) through the network. So far, so good.
You have a website set up in IIS which has an association with a set of IP addresses. For the purposes of this example, lets say that the site is only associated with 123.45.678.90. You have DNN installed in that website. We will assume that the IP association is a wildcard, that is that in the configuration... the site is set to answer all requests to the IP address 123.45.678.90. I point that out because some hosts actually include your url in the association which would prohibit it from answering other url's without another entry specific for it.
Getting URL's to the DNN site:
Simply set the DNS associated with the URL to point to 123.45.678.90. This can be done for any number of sites. In practice, I usually set three A records ( www.domain1.com, domain1.com and *.domain1.com ). This provides for the ability to add cname sites ( instead of child sites ) using the format "cname.domain1.com".
Getting the URL's to a specific portal is simply a matter of setting the appropriate Portal Alias's in the Site Settings ( under a Super User account ).
That is all there is to it. You can associate multiple IP addresses if you wish, but there is no requirment to do so. Once you understand these basic mechanics, you can figure out how to tweak your environment the way you want it, everyone does something a little different.
Implications
Virtually none. MX records are for eMail and really don't have anything to do with your website.
You'll develop additional questions later based on how you want to do things, what software you use for other purposes, unique aspects of your configuration, etc. But out of the box, it really is that simple.
Cheers,
Scott