Mil Used for U.S.military organizations.
Int International organizations.
Net Used for Network organizations such as Internet service providers.
Org Used for nonprofit organizations.
arpa Used for inverse address lookups.
The structure of the Domain Name System is similar to an inverted tree.
At each level,a fully-qualified domain name(FQDN)is created by concatenating the local name with the names of the entities above it in the hierarchy.Thus madn.Microsoft.com is used to name the madn subdomain in the Microsoft domain that falls under the com domain.By using the FQDN.For example,fileserver.twoinc.com names a host called fileserver.This host cannot become confused with another host of the same name that resides in a different domain such as fileserer.acme.com.
There are a number of restrictions to the names you can use in the DNS system:
The maximum length of a domain name or a host label is 63 characters.
The maximum length for the FQDN is 255 characters.
There can be up to 127 subdomains.
All text is not case sensitive.
Prinmary ,Secondary,and Caching-Only Name Servers
For each domain on the Internet there must be a primary server and a secondary server.The prinmary DNS server for the domain contains a collection of resource records that contain the address mappings for hostnames in the domain.The primary DNS server is the final authority for these mappings.The secondary DNS server contains a copy of the database maintained by the pinmary server and can continue to resolve the name when the primary server is offline.It is important to note that the pinmary DNS server is where changes are made to the database.Through the use of the zone transfer mechanism,the data is copied to secondary servers.
In many cases,a DNS server answers name queries for domains for which it is not the suthority.In this case the DNS server contacts a DNS server further up the hhierarchy until one is found that can resolve the name translation or that can point to another DNS server that is the authority for the name.The DNS server maintains a cache of names that have been resolved by this method so that it does net have to continually poll other servers for names that are frequently queried.
A third type of DNS server is a caching-only server.This type of server does not maintain a database for a particular zone.To put it in other terms,it is not authoritative for any zone or domain and does not use the zone transfer mechanism to keep a current copy of the entire database.Instend,a caching-only name server has to contact another DNS server to initially resolve a name.but like the other servers,it maintains a cache of names it has resolved so that it does not have to keep forwarding the query to another server.This type of server is usually used on a network segment that is connected to the rest of the network by a slower link or a more expensive one and is used to reduce network traffic.
Broadband access technologies
Broadband access technologies hava been spawned by the efforts of the telecommunications and video networking industries to move toward convergence at the physical layer.By contrast,the efforts of the data networking industry toward convergence hava generally focused on the networking and transport layers(e.g.,IP telephony).The reason for this difference in approach is that,once content has been digitized,it is treated as data.In other words,form a networking perspective,the network merely transports data in digital format.It makes no difference whether the bits actually represent voice,data,or videostreams at the application level.Consequently,the data net working industry did not have to do anything different to adapt a data stream for transport across a network.On the other hand,the telecommunications and bideo net working industries had to figure out way to transport their content as packetized data cost-effectively by working with existing access infrastructure.This was an important catalyst in the development of the following classes of broadband access technologies:
Copper-loop access technologies.also known as digital subscriber line (DSL)technologies.DSL technologies are collectively referred to as XDSL.XDSL was developed by the telecommunications industry to make use of the several million miles of existing copper loop infrastructure around the world.
Cable access technologies over fiver/coaxial cable.Cable access was developed by the cable TV portion of the video networking industry to take advantage fo the cable infrastructure that feeds viedo channels to several million(mostly residential)subscribers around the world.
Satellite access technologies over wireless medium.Satellite access was developed by the wireless portion of the video net working industry to make use of the satellite infrastructure that feeds video channels to cable TV head end offices and to several hundred(mostly residential)subscribers around the world.
ADSL
ADSL was first developed in the 1980sas the telecommunications industry’s answer to the cable industry’ repuest to support video on demand.In the mid 1990s,however,it was quickly recognized as a viable technoloty to enable access to hight-speed services such as the Interne.
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