This is just a question of curiosity. Let's say I register for a new web domain and purchase a hosting account. I set up a website, and soon search index bots are visiting the site to learn more about it. How did those bots know how to find the site if I haven't linked to it from anywhere else on the web?
My guess would be that the search providers obtain new IPs from either the domain name registrar or the hosting provider and feed these to their spiders, but I've been unable to find a clear answer on this.