At the top of the domain name hierarchy is the root domain (typically references by a single dot, or <root>). Information about this domain resides on 13 root DNS servers located around the world.
All Internet DNS servers are configured with references to these root servers referred to as the "root file", "hints file" or "cache file".
Below the root domain are the top-level domains, which are either country specific or generic. Examples of country specific top-level domains are SG (Singapore) and CA (Canada), while generic top-level domains include the well-known COM (commercial organizations), EDU (educational institutions), GOV (governmental organizations), and NET (network organizations), among others. Note that top-level domains outside the U.S. are typically country specific, while U.S.-based sites typically use generic names. Below the top-level domains are the second-level domains (whitehouse.gov, microsoft.com, simpledns.com), and then the third-level domains, and so on down the chain.
To locate any domain name, a DNS server generally starts by asking one of the root servers (unless it already has a closer match cached)
The root server will supply references (NS-records) to DNS servers responsible for the next level (.com, .net, etc.).
The DNS server then repeats the request to one of those server, which will supply references to the next level (for example simpledns.com), and so it goes on until the requested domain name is found.
This process is know as recursion.
This way a DNS server can locate any name in the world, as long as it knows the addresses of the root DNS servers.
Simple DNS Plus includes the standard root file ("named.root") from the InterNIC containing records for the current Internet root DNS servers. This file is automatically loaded at startup (unless a private root zone has been defined), and Simple DNS Plus automatically checks for updates to keep it current (unless you disable this using the "AutoUpdateRoot" option in the sdnsplus.ini file)
