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In computer networking, reverse DNS lookup or reverse DNS resolution (rDNS) is the determination of a domain name that is associated with a given IP address using the Domain Name System (DNS) of the Internet. Computer networks use the Domain Name System to determine the IP address associated with a domain name. This process is also known as ''forward'' DNS resolution. ''Reverse'' DNS lookup is the inverse process, the resolution of an IP address to its designated domain name. The reverse DNS database of the Internet is rooted in the ''Address and Routing Parameter Area'' (arpa) top-level domain of the Internet. IPv4 uses the in-addr.arpa domain and the ip6.arpa domain (previously ip6.int〔https://tools.ietf.org/html/rfc4159〕) is delegated for IPv6. The process of reverse resolving an IP address uses the ''pointer'' DNS record type (PTR record). Informational RFCs (RFC 1033, RFC 1912 Section 2.1) specify that ''"Every Internet-reachable host should have a name"'' and that such names match with a reverse pointer record, but it is not a requirement of standards governing operation of the DNS itself. ==IPv4 reverse resolution== Reverse DNS lookups for IPv4 addresses use a ''reverse IN-ADDR entry'' in the special domain in-addr.arpa. In this domain, an IPv4 address is represented as a concatenated sequence of ''four decimal numbers'', separated by dots, to which is appended the second level domain suffix .in-addr.arpa. The four decimal numbers are obtained by splitting the 32-bit IPv4 address into four 8-bit portions and converting each 8-bit portion into a decimal number. These decimal numbers are then concatenated in the order: least significant 8-bit portion first (leftmost), most significant 8-bit portion last (rightmost). It is important to note that ''this is the reverse order to the usual dotted-decimal convention for writing IPv4 addresses'' in textual form. For example, an address (A) record for mail.example.com points to the IP address 192.0.2.5. In pointer records of the reverse database, this IP address is stored as the domain name 5.2.0.192.in-addr.arpa pointing back to its designated host name mail.example.com. This allows it to pass the Forward Confirmed reverse DNS process. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Reverse DNS lookup」の詳細全文を読む スポンサード リンク
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