No and Yes.
Having the subdomain available will make it easier but the key thing is to understand the portal alias concept. When you have one install of dnn serving several portals it is the alias that determines which portal a url wil go to. It also allows you to have several different urls going to the same portal as there can be multiple entries in the alias field.
For example, any url (domain) will resolve to an IP address. A subdomain will also resolve to the same IP address. It is up to the website software to sort out the difference and this is how dnn works.
Lets say you have a webserver set up with an install of dnn holding two portals. On portal is for the main site and the second portal (I prefer parent portals but it could be a child portal) is for a subdomain. The server is on ip address 192.0.0.1 (just an example). Domain is myowndomain.com and subdomain is gavin.myowndomain.com.
Now, in the alias for the main site you have the entry www.myowndomain.com. This directs all requests to www.myowndomain.com to the main site. In the alias field for the sub-portal you have www.gavin.myowndomain.com. Therefore, all requests for www.gavin.myowndomain.com get directed to the second or sub-portal. Following along?
Now, remember that both the domain and the subdomain both point to the same ip address? Well, you could put that ip address, 192.0.0.1 in this example, into either of the alias fields depending on which portal you wanted the ip address to go to. This is how you can access the sub-domain site without the subdomain being all set up in dns etc.
To actually set the aliases you can do it in the database (not recommended) in the portals table. Preferably, you can do it through the dnn interface. Log in as host and go to the host > portals tab and set them in there. The format is a comma seperated list with no spaces.
In reality you will usually have two aliases i.e www.myowndomain.com,myowndomain.com so people can use or not use the 'www.' part. Similarly, in the subdomain portal alias you would have www.gavin.myowndomain.com,gavin.myowndomain.com
Just remember that with domains and subdomains they all resolve to the same IP address.
Sorry if this is a big longwinded but I thought a long explaination might help others.
Corrections anyone?