Root Servers:
Root server -- these servers are at the base of the name server hierarchy. They are a fixed set of name servers that maintain a list of the authoritative (master/slave) name servers for every registered domain. These are typically name servers located at companies either contracted to provide the service by ICANN or government institutions. See the list at
http://www.root-servers.org/ . The 13 root name servers are operated by 12 independent organizations.
Hosting Provider or ISP DNS:
Authoritative server -- a master/slave [ns1,ns2]server for a particular domain that has been configured by an administrator with the hostname information for that domain. Information about these servers is added to the root servers when the domain is registered.
ISP or Public Google DNS:
Local (caching/forwarding) server -- a local name server that only caches information for local clients once it has been retrieved from an authoritative name server. The local server can effectively speed up name queries for the local network by serving up names found by prior queries, preventing a request to the authoritative server for that host's domain.
rDNS / PTR: Reverse DNS, check host-name by IP. Set PTR records for IP so that it will show a host-name rather IP.
