TCP/IP networks rely on IP addresses to keep track of all the devices on the network. Computers can keep track of thousands of IP addresses, but people more easily remember names.
A DNS server is a database that translates IP addresses into user-friendly and portable names. e.g. ftp.microsoft.com instead of numeric address like 188.8.131.52.
At the infancy stage of the Internet, there weren”t very much options for keep track of IP addresses. Third party DNS server software and manual entries in HOSTS files were the few options available.
InterNIC was formed as more and more devices are connected to the Internet the need for a central clearinghouse for domain names was required. The 13 root servers addresses the needs for resolving all TLD (top-level domains) namely EDU, NET, COM, MIL, ORG, GOV. Country code TLD were later on introduced in 1985 which cover country specific domains like .sg for Singapore (as in domain.com.sg), .my for Malaysia and so forth.
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While the root server holds the information about the authoritative DNS servers for a particular zone (a domain name space), the authoritative name servers holds the complete information about one part of a zone. Thus it is said to be the authoritative for that zone. It is common for people to get confused and think that a domain name is computer.com, which is only partially correct. In reality the domain is .COM and the authoritative zone is computer.
At each node (a point where further branching occurs, or where periods occur in the name) there is a name server for the remainder of that branch down which is managed by a name server called a “zone of authority”. A single name server can be in charge of different zones at the same time.
There are normally two or more authoritative DNS servers for a particular domain name space as backups in case the primary DNS server is down.
The secondary master and all backup name servers get their data from the primary master. The information is passed using a zone transfer whereby all information about a particular domain are transferred and stored in the secondary periodically.
In terms of ISP DNS servers, primary DNS and secondary DNS are not queried in descending order although it may seems so.