Data Communications Networking Devices - 4th Ed.pdf

2.5 THE INTERNET _______________________________________________________________ 223Figure 2.32A domain-naming hierarchySince modern operating systems can support multiple applications, anothercommon method of structuring domain names is based upon the use of the followingformat:application:organization:toplevel domainHere applications can range from ftp and telnet servers to gopher and World WideWeb WWW) servers. The organization can represent any registered entity, fromApple to IBM to Microsoft, while the top-level domain would be one of thedomain names previously listed in the left-hand column of Table 2.5.Although domain names provide a mechanism for identifying objects connectedto wide area networks, hosts in a domain require network addresses to transferinformation. Thus another host functioning as a name server is required to providea name to address translation service.Name serverThe name server plays an important role in TCP/IP networks. In addition toproviding a name-to-IP address translation service it must recognize that anaddress is outside its administrative zone of authority. For example, assume ahost located on the domain illustrated in Figure 2.32 will use the addressfred.microwear.com to transmit a message. The name server must recognize thatthat address does not reside in its domain and must forward the address to anothername server for translation into an appropriate IP address. Since most domains areconnected to the Internet via an Internet service provider, the name server on thedomain illustrated in Figure 2.32 would have a pointer to the name server of theInternet Service Provider and forward the query to that name server. The InternetService Provider's name server will either have an entry in its table in cachememory or forward the query to another higher level name server. Eventually a