The Domain Name System (DNS) is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. The purpose of the DNS database is to translate computer names into IP addresses. In DNS clients are called resolver and servers are called name servers. A host computer contacts a computer name where upon DNS server

connects a name to an IP address. The resolver first uses UDP to contact the server

A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet. The top-level domain names are installed in the root zone of the name space. For all domains in lower levels, it is the last part of the domain name, that is, the last label of a fully qualified domain name. For example, in the domain name www.example.com, the top-level domain is com. Responsibility for management of most top-level domains is delegated to specific organizations by the Internet Corporation for Assigned Names and Numbers (ICANN), which operates the Internet Assigned Numbers Authority (IANA), and is in charge of maintaining the DNS root zone.

When a DNS client needs to look up a name used in a program, it queries DNS servers to resolve the name. Each query message the client sends contains three pieces of information, specifying a question for the server to answer:

A specified DNS domain name, stated as a fully qualified domain name (FQDN)

A specified query type, which can either specify a resource record by type or a specialized type of query operation

A specified class for the DNS domain name. 

In general, the DNS query process occurs in two parts:

1) A name query begins at a client computer and is passed to a resolver, the DNS Client service, for resolution.

2) When the query cannot be resolved locally, DNS servers can be queried as needed to resolve the name.

When the authoritative name server for a domain receives a request for a subdomain's records and responds with NS records for other name servers, that is DNS delegation. Essentially it is saying "I am passing on authority for this subdomain to another collection of name servers, go ask them for the details."

Example: Delegating a subdomain to a new zone

As shown in the following figure, when a new zone for a sub domain (example.microsoft.com) is created, delegation from the parent zone (microsoft.com) is needed.

In this example, an authoritative DNS server computer for the newly delegated example.microsoft.com subdomain is named based on a derivative subdomain included in the new zone (ns1.us.example.microsoft.com). To make this server known to others outside of the new delegated zone, two RRs are needed in the microsoft.com zone to complete delegation to the new zone. An NS RR to effect the delegation. This RR is used to advertise that the server named ns1.us.example.microsoft.com is an authoritative server for the delegated subdomain.

DNS records are basically mapping files that tell the DNS server which IP address each domain is associated with, and how to handle requests sent to each domain. When someone visits a web site, a request is sent to the DNS server and then forwarded to the web server provided by a web hosting company, which contain the data contained on the site.

Various strings of letters are used as commands that dictate the actions of the DNS server, and these strings of commands are called DNS syntax.  Some DNS records syntax that are commonly used in nearly all DNS record configurations are A, AAAA, CNAME, MX, PTR, NS, SOA, SRV, TXT, and NAPTR.

The Domain Name System (DNS) is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. 

Reverse DNS is IP address to domain name mapping - the opposite of forward (normal) DNS which maps domain names to IP addresses. Forward DNS for "abc.com" pointing to IP address "1.2.3.4", does not necessarily mean that reverse DNS for IP "1.2.3.4" also points to "abc.com" Reverse DNS is mostly used by humans for such things as tracking where a web-site visitor came from, or where an e-mail message originated etc. It is typically not as critical in as forward DNS - visitors will still reach your web-site just fine without any reverse DNS for your web-server IP or the visitor's IP.

Any answer that originates from the DNS Server which has the complete zone file information available for the domain is said to be authoritative answer.

In many cases, DNS servers will not have the complete zone file information available for a given domain. Instead, it maintains a cache file which has the results of all queries performed in the past for which it has gotten authoritative response. When a DNS query is given, it searches the cache file, and return the information available as “Non-Authoritative Answer”. Non authoritative servers do not contain copies of any domains. Instead they have a cache file that is constructed from all the DNS lookups it has performed in the past for which it has gotten an authoritative response. 

Domain name resolver are the names given to computers, commonly located with Internet Service Providers (ISPs) or institutional networks that are used to respond to a user request to resolve a domain name. These computers translate a domain name into an IP address. Also called DNS resolvers.

A DNS cache contains entries that translate Internet domain names *such as "compnetworking.about.com") to IP addresses. The Internet's Domain Name System (DNS) involves caching on both Internet DNS servers and on the client computers that contact DNS servers. These caches provide an efficient way for DNS to efficiently keep the Internet synchronized as the IP addresses of some servers change and as new servers come online.

As your machine gets started, it will need to know the mapping of some hostnames to IP addresses before DNS can be referenced. This mapping is kept in the /etc/hosts file. In the absence of a name server, any network program on your system consults this file to determine the IP address that corresponds to a host name.

The leftmost column is the IP address to be resolved. The next column is that host's name. Any subsequent columns are alias for that host. In the second line, for example, the IP address 208.164.186.1 is for the host deep.openna.com. Another name for deep.openna.com is deep. After you are finished configuring your networking files, don't forget to restart your network for the changes to take effect.