Reverse lookup is a network service used to determinate a domain name that is associated with a given IP address using the Domain Name System (DNS) of the Internet. It takes the form of a question, such as “Can you tell me the DNS name of the computer that uses the IP address 192.168.1.1?”
One problem for supporting the reverse query process is the difference in how the DNS namespace organizes and indexes names and how IP addresses are assigned. If the only method to answer the previous question was to search in all domains in the DNS namespace, a reverse query would take too long and require too much processing to be useful.
To solve this problem, a special domain, the in-addr.arpa domain, was defined in the DNS standards and reserved in the Internet DNS namespace to provide a practical and reliable way to perform reverse queries. To create the reverse namespace, subdomains within the in-addr.arpa domain are formed using the reverse ordering of the numbers in the dotted-decimal notation of IP addresses. This reversed ordering of the domains for each octet value is needed because, unlike DNS names, when IP addresses are read from left to right, they are interpreted in the opposite manner.
When an IP address is read from left to right, it is viewed from its most generalized information (an IP network address) in the first part of the address to the more specific information (an IP host address) contained in the last octets. For this reason, the order of IP address octets must be reversed when building the in-addr.arpa domain tree. An seoltaí IP de na DNS-addr.arpa is féidir le crann a tharmligean chuig comhlachtaí mar a bhfuil siad sannta ar leith nó sraith teoranta de seoltaí IP laistigh de na Idirlíon-shainithe seoladh ranganna. Ar deireadh, an i-addr.arpa fearainn crann, mar a tógadh isteach DNS, éilíonn sin acmhainní breise a thaifeadadh (RR) type the pointer (PTR) RR a bheith sainithe. Seo a RR a úsáidtear chun a chruthú le mapáil i droim ar ais lookup crios go bhfuil de ghnáth freagraíonn a lán (A) ainmnithe RR le haghaidh an DNS ríomhaire ainm óstach ina aghaidh lookup crios.
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