You might be thinking about Private domain registrations such as what GoDaddy Provides. DNS information is meant to be shared in order for people to be successfully forwarded to sites/services you want them to reach. In order for people to reach you, they need to know one or two things: DNS name or an IP.
Through DNS, anyone can find information about who's hosting what for your domain, just do a quick DNS lookup or go to http://www.intodns.com/, and find a bunch of sweet DNS information about a domain.
IP owners can be found doing a quick WHOIS lookup since ARIN keeps a public database of every address it allocates. They HAVE TO be able to provide administrative contact information. There is no way to make this private.