Originally, computers performed name-to-address mapping via a simple text file, the hosts file, which contained a list of every machine that needed to be referenced by name.
Using the hosts file, a computer could resolve a lookup. Every computer had a copy of the hosts file. If an IP address changed for any machine in that file, the reference would need to be changed in the hosts file and every computer’s hosts file would need to be updated to reflect the change. Clearly, the number of machines on the Internet today makes this an impossible task. The Domain Name System overcomes the limitations presented by the hosts file scheme.
DNS is a distributed database, allowing local control over portions of the database. At the top of the Domain Name System hierarchy, about 13 root name servers point the way to other DNS servers responsible for a generic top-level domain (gTLD), such as “.com.” Root servers are located at high-bandwidth points around the world. These servers in turn point the way to the authoritative server for the query the server listed with the registrar that can answer queries with authority. This hierarchy is shown in the following illustration.
When you register a domain name with a registrar, you must also define the authoritative DNS servers for the domain the servers that the root servers will ultimately send queries to for your domain.
The major registrars tend to provide DNS service for domains registered with them. The mere act of setting up a DNS server does not cause outside entities to suddenly query it general queries from the Internet will always use the authoritative servers defined by your registrar via the root servers.
Turning on the DNS service in Mac OS X Server behind your firewall affects only your local network, as shown in the next illustration of a DNS setup.