This isn't really a DNN Framework issue, ASP .NET, which the framework is based on, has many options for this but DNN may have a better way that I haven't found yet.  To find the domain for any website you can use the code below.  This will only return the domain and not the entire url, the problem come up if you have a child portal as this won't return the child portal portion of the address.  If you need the entire url then I can show you how to do that but it takes a bit more code then is easily shown here.  Also, this is in C#, I do have it in vb but it isn't hard to translate. 

To Find Domain:
string partialUrl = Request.Url.GetLeftPart(UriPartial.Authority);