It's quite a few years since I've used Dynamic DNS. As far as I remember, the provider (?) I used in those days provide a choice of top-level domains, and users were invited to choose an (unused) subdomain to associate with their router's IP. (I've come to ZoneEdit because my router supports it.). But all I can see is how to associate DDNS with an existing domain. So are we supposed to register a domain just for this use? I'm confused!