You will need some kind of domain (or sub-domain) to actually host the site here on DreamHost. You do not have to transfer the domain registration, you can simply configure the domain to point at the DreamHost name servers and leave it registered at your current registrar, or if there is some reason why you can not use this domain just yet, you can make use of a free dreamhosters.com sub-domain.
As for accessing the site by IP address; This can't really be done unless you purchase a unique IP address, as the IP addresses normally allocated to domains and sub-domains are shared among customers and there may be dozens of sites on a particular IP address. There is a method, involving the modification of your computers 'hosts' file, to map a domain name to the (shared) IP address allocated to your site, but this would only work on those systems where you have made the required changes to the 'hosts' file.
If you simply wish to prevent the search engines, or anyone else, from accessing the site while it is in development, there are easier methods. Probably the easiest method would be to create a simple .htaccess file limiting access to your own IP address (or IP address block). You can also easily configure username/password access for your site, or parts of the site, using the .htaccess file.
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