tailieunhanh - Internetworking with TCP/IP- P53

Internetworking with TCP/IP- P53: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | Sec. Pointer Queries 479 Pointer Queries One form of inverse mapping is so obviously needed that the domain system supports a special domain and a special form of question called a pointer query to answer it. In a pointer query the question presented to a domain name server specifies an IP address encoded as a printable string in the form of a domain name . a textual representation of digits separated by periods . A pointer query requests the name server to return the correct domain name for the machine with the specified IP address. Pointer queries are especially useful for diskless machines because they allow the system to obtain a high-level name given only an IP address. We have already seen in Chapter 6 how a diskless machine can obtain its IP address. Pointer queries are not difficult to generate. If we think of an IP address written in dotted-decimal form it has the following format To form a pointer query the client rearranges the dotted decimal representation of the address into a string of the form ddd. ccc. bbb. aaa. in-addr. arpa The new form is a name in the special domainf. Because the local name server may not be the authority for either the arpa domain or the domain it may need to contact other name servers to complete the resolution. To make the resolution of pointer queries efficient the Internet root domain servers maintain a database of valid IP addresses along with information about domain name servers that can resolve each address. Object Types And Resource Record Contents We have mentioned that the domain name system can be used for translating a domain name to a mail exchanger address as well as for translating a host name to an IP address. The domain system is quite general in that it can be used for arbitrary hierarchical names. For example one might decide to store the names of available computational services along with a mapping from each name to the telephone number to call