What is DNS or Domain Name System

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The  Domain Name System (DNS) is a hierarchical and distributed name management system for computers, services, or any feature connected to the Internet or a private network. It is based on hierarchical names and allows the registration of several data entered in addition to the host name and its  IP . Because the DNS database is distributed, its size is unlimited, and performance does not degrade greatly when more servers are added. This type of server uses  53 as  default port .
The DNS-Berkeley implementation was originally developed for the BSD UNIX 4.3 operating system.  The Microsoft DNS Server implementation has become part of the Windows NT operating system in the Server 4.0 release. DNS became the default name resolution service from Windows 2000 Server as most DNS implementations had their roots in RFCs 882 and 883 and were updated in RFCs 1034 and 1035.
An Internet resource, for example a Web site, can be identified in two ways: by its domain name, for example, “www.facebook.com” or by the IP address of the hosts hosting it (for example, 208.80 .152.130 is the IP associated with the domain www.facebook.com). IP addresses are used by the network layer to determine the physical and virtual location of the equipment. Domain names, however, are more mnemonic for the user and business. A mechanism is then required to translate a domain name into an IP address. This is the primary function of DNS.
Occasionally, it is assumed that DNS serves only the purpose of mapping Internet host names to data and map addresses to host names. This is not correct, DNS is a hierarchical (though limited) database, and can store almost any type of data, for almost any purpose.
Because of the size of the Internet, storing all domain-IP address pairs on a single DNS server would be impractical for scalability issues that include:
Reliability: If the single DNS server failed, the service would become unavailable to the entire world.
Traffic volume: The server should handle the DNS requests of the entire planet.
Distance: Most users would be far away from the server, wherever it was installed, generating long delays to resolve DNS requests.
Database maintenance: The database should store a huge amount of data and would have to be updated with a very high frequency every time a domain was associated with an IP address.
The solution is to make DNS a distributed and hierarchical database. The DNS servers fall into the following categories:
Root Servers
At the top of the hierarchy are the 13 root servers. A root server is a name server for the Domain Name System (DNS) root zone. Its function is to respond directly to requests from root zone records and respond to other requests by returning a list of name servers assigned to the appropriate top-level domain. Root servers are a crucial part of the Internet because they are the first step in translating names to IP addresses and are used for communication between hosts.
Top-level domain servers
Each domain consists of names separated by periods. The rightmost name is called the top domain. Examples of top domains are .com, .org, .net, .edu, .inf and .gov.
Each top-level domain server knows the addresses of the authoritative servers that belong to that top-level domain, or the address of some intermediate DNS server that knows an authoritative server.
Reverse DNS
Normally, DNS acts by resolving the domain name of any host to its corresponding IP address. Reverse DNS resolves the IP address by searching for the domain name associated with the host. That is, when we have available the IP address of a host and we do not know the address of the domain (name given to the machine or other equipment that accesses a network), we try to solve the IP address through reverse DNS that looks for which domain name is associated Address. Servers that use Reverse DNS can verify the authenticity of addresses by verifying that the current IP address matches the IP address entered by the DNS server. This prevents someone from using a domain that does not belong to them to send spam, for example. Obtaining Domain Names and IP Addresses
Domain namespace and IP addresses are critical resources for the internet, in the sense that they require global coordination. Each IP address must identify a single device, so that it is not possible to assign IP addresses in a decentralized manner. Similarly, a domain name must identify the set of computers that maintains it. The organization responsible for assigning domain names and IP addresses globally is ICANN.
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