Domain Name System Detail description
Domain Name System Detail description
To identify an entity, the internet uses the IP address, because to uniquely identify particular host on internet and World Wide Web it requires IP address, which identifies the connection of a computer to the internet. But user prefers to use names instead of IP address because it is numeric address and to remember this numeric address is more difficult compare to names. Also, sending e-mail to particular user e.g cedept@192.168.4.254 means that if cedept’s ISP or organization moves the mail server to a different machine with a different IP address that means e-mail address has to change. Consequently, if ASCII names used to decouple machine name from machine addresses, sspc’s address might be something like that cedept@sspc.ac.in. But the network itself understands only numerical addresses. Therefore we need a system that can map a name to an address or an address to a name. The mapping scheme used in the internet is called the DNS (Domain Name System). DNS is the invention of a hierarchical, domain-based naming scheme and a distributed database system for implementing this naming scheme. It is primarily used for mapping host names and e-mail destinations to IP addresses but can also be used for other purposes. The Domain Name System (DNS) is system for computers or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participants. Most importantly, it translates domain names meaningful and easily remembered by humans into the numerical (binary) identifiers associated with networking equipment for the purpose of locating and addressing these devices worldwide. For example Domain Name System is that it serves as the “phone book” in which user cannot remember the other’s contact number, so they save numbers by his/her name. Unlike the phone book, DNS translates human-friendly computer hostnames into IP addresses. For example, www.sspc.ac.in translates to 192.168.4.254. IN DNS names must be unique because the addresses are unique. Domain names are case insensitive. For example Com, COM, com these all are have same meanings. DNS maps to level 7 in the OSI model and can use either UDP or TCP as the underlying protocol. In general, Domain names and Host names have restrictions in their naming that only allow the use of characters “a-z,” “A-Z,” “0-9,” and “-” (dash or minus sign). The use of characters such as the “/,” “.,” and “_” (slash, period, and underscore) are not allowed. The Domain Name System (DNS) is a set of protocols and services on a TCP/IP network that allows users of the network to utilize hierarchical user-friendly names when looking for other hosts (that is, computers) instead of having to remember and use their IP addresses. This system is used extensively on the Internet and in many private enterprises today. If you’ve used a Web browser, Telnet application, FTP utility, or other similar TCP/IP utilities on the Internet, then you have probably used a DNS server. The DNS protocol’s best-known function is mapping user-friendly names to IP addresses. For example, suppose the FTP site at Microsoft had an IP address of 157.55.100.1. Most people would reach this computer by specifying FTP.microsoft.com and not the less “friendly” IP address. Besides being easier to remember, the name is more reliable. The numeric address could change for any number of reasons, but the name can always be used. To map a name into an IP address, an application program calls a library procedure called the Resolver, passing its name as parameter. The resolver sends a UDP packet to a local DNS server, which then looks up the name and returns the IP address to the resolver, which then returns it to the caller process. When you request a domain from any program such as a web browser or ftp client or send an email, that request goes to a DNS server (your isp’s dns server first) to find out what the machine address is for that domain. If that first DNS server doesn’t know the machine address (called an ip address) then it will ask the DNS server above it and so on until a DNS server that knows the ip address of the domain you want can reply back with the appropriate ip address. See the example in below figure.
DNS names are defined in an inverted tree structure with the root at the top. Each node in the tree has a domain name. Each node in the tree has a label, which is string with maximum of 63 characters. A domain name is the sequence of labels from a node to the root, separated by dots (“.”s).left to right. Domain names are always read from the bottom to top. The name space has a maximum depth of 127 levels and Domain names are limited to 255 characters in length. 
[Domain Name Space]
Each domain is partitioned into sub domains and these are further partitioned and so on. For example, challenger.atc.fhda.edu is a sub domain of atc.fhda.edu, in turn it is sub domain of fhda.edu, and again in turn it is sub domain of edu. See below figure.
[Domain names and labels]
In the internet, domain name space is divided into three different categories:1. Generic Domain
2. Country Domain
3. Inverse Domain
Generic Domain: – Generic domain defines register hosts according to their generic behavior. Each node in the tree defines a domain. A generic domain (gTLD) is one of the categories of top-level domains (TLDs) maintained by the Internet Assigned Numbers Authority (IANA) for use in the Domain Name System of the Internet. A well known generic domains are:com – Commercial organizations biz – Business or firms
edu – Educational institutions gov – Government institutions
info – Information service provider int – International organizations
mil – Military groups net – Network organizations
org – Nonprofit organization aero – Airlines & Aerospace companies
coop – Cooperative business org. name – Personal names (individuals)
pro – Professional individual org. mobi – Mobile electronics
Country Domain:- Country domain defines country’s name and uses two character country abbreviation in place of three or four character abbreviation in generic domains. Country domain names are sometimes referred to as Country Code TLDs (Top Level Domains). Essentially Country Domain Names are domains designated to each country (i.e. United States, United Kingdom, Germany, Canada, etc.). These domain name extensions often have specific requirements enforced on them by their respective Registries. For example, the .US Registry (www.nic.us) maintains that .US domains must be registered by a United States Citizen.
The Country domains are:
in – India us – Unites State ca – Canada
au – Australia jp – Japan uk – United Kingdom
nz – New Zeland cn – China tw – Taiwan
fr – France ae – United Arab Emirates zw – Zimbabwe etc.
Inverse Domain: – A Domain Name System is basically used to map host name to IP address. But in Inverse domain, it is used to map an address to a host name. When a server has received a request from a client to do a task, whereas server has a file that contains a list of authorized clients, server lists only the IP address of the client. To determine if the client is on authorized list, the server can send a query to the inverse DNS server and ask for a mapping of address to name. Below figure shows the inverse process. The inverse process is added to the domain name space with the first-level node called arpa (Advanced Research Project Agency ). The second level is also one single node named in-addr. (for inverse address) The rest of the domain defines IP address.
An IP address such as 132.34.45.121 (class B address) this is read as 121.45.34.132 in-addr arpa.