DNS is a protocol that can be used in different platforms. in the Internet, the Domain name space is divided into three different sections: generic domains, country domains, and inverse domain.
Generic Domain
The generic domain define registered hosts according to their generic behavior. Each node in the tree defines a domain, which is an index to the domain name space database.
First level in generic domain section allows seven possible three-character labels (or extensions). These labels describes the organization types as listed:
recently a few more first-level domain names heve been approved:
Country Domain:
The country domain uses the same format as generic domains but uses two-character country abbreviations (eg., .us fro United States, .in for India), in place of three-character organizational abbreviations at the foiat level. Second-Level labels can be organizational, or they can be more specific, national designations. The United States, For example, uses state abbreviations as a subdivision of us (e.g., .ca.us.).
Inverse Domain
The inverse domain is used to map an address to a name. This may happen,, for example, when as server has received a request from client to do a task. Whereas the server has a file that contains a list of authorized clients, the serve lists only the Ip address of the client (extracted from received IP packet). To determine if the client is on the authorized list, it can ask its resolver to send a query to the DNS server and ask for a maping of address to name.
This type of query is called an inverse or pointer (PTR) query. To handle a pointer query, the inverse domain is added to the domain name space with the first-level node called arpa ( for historical reasons). the second level is also one single node named in-addr (for inverse address). The rest of the domain defines IP addresses.
The servers that handle the inverse domains are also hierarchichal. this means the netid part of the address should be at higher level than the subnetid part, and the subnetid part higher than the hostid part. In this way, a server serving the whole site is at a higher level than the servers serving each subnet. this configuration makes the domain look inverted when compared to a gneric or country domain.
Example of Inverse Domain
To follow the convention of reading the domain names from the bottom to the top, an IP address such as 132.34.45.121 9a class B address with netid 132.34) is read as 121.45.34.132.inaddr.arpa.
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Generic Domain
The generic domain define registered hosts according to their generic behavior. Each node in the tree defines a domain, which is an index to the domain name space database.
First level in generic domain section allows seven possible three-character labels (or extensions). These labels describes the organization types as listed:
| Label | Description |
| .com | Commercial organizations |
| .edu | Educational Institutions |
| .gov | Government Institutions |
| .int | International organizations |
| .Mil | Military groups |
| .net | Network support centers |
| .org | Nonprofit organizations |
recently a few more first-level domain names heve been approved:
| Label | Description |
| .aero | Airlines and aerospace companies |
| .biz | Business or firms (similar to ".com") |
| .coop | Cooperative Business Organization |
| .info | Information service providers |
| .museum | Museums and other non profit organizations |
| .name | Personal names (individuals) |
| .org | Nonprofit organizations |
| .pro | Professional Individual organizations |
Country Domain:
The country domain uses the same format as generic domains but uses two-character country abbreviations (eg., .us fro United States, .in for India), in place of three-character organizational abbreviations at the foiat level. Second-Level labels can be organizational, or they can be more specific, national designations. The United States, For example, uses state abbreviations as a subdivision of us (e.g., .ca.us.).
Inverse Domain
The inverse domain is used to map an address to a name. This may happen,, for example, when as server has received a request from client to do a task. Whereas the server has a file that contains a list of authorized clients, the serve lists only the Ip address of the client (extracted from received IP packet). To determine if the client is on the authorized list, it can ask its resolver to send a query to the DNS server and ask for a maping of address to name.
This type of query is called an inverse or pointer (PTR) query. To handle a pointer query, the inverse domain is added to the domain name space with the first-level node called arpa ( for historical reasons). the second level is also one single node named in-addr (for inverse address). The rest of the domain defines IP addresses.
The servers that handle the inverse domains are also hierarchichal. this means the netid part of the address should be at higher level than the subnetid part, and the subnetid part higher than the hostid part. In this way, a server serving the whole site is at a higher level than the servers serving each subnet. this configuration makes the domain look inverted when compared to a gneric or country domain.
Example of Inverse Domain
To follow the convention of reading the domain names from the bottom to the top, an IP address such as 132.34.45.121 9a class B address with netid 132.34) is read as 121.45.34.132.inaddr.arpa.
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