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Classless Inter-Domain Routing (CIDR / ˈ s aɪ d ər, ˈ s ɪ-/) is a method for allocating IP addresses for IP routing. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous classful network addressing architecture on the Internet .
The modern "reverse DNS lookup" should not be confused with the now-obsolete "inverse query" (IQUERY) mechanism specified in RFC 1035: Inverse queries take the form of a single resource record (RR) in the answer section of the message, with an empty question section. The owner name of the query RR and its time to live (TTL) are not significant.
Some large / 8 blocks of IPv4 addresses, the former Class A network blocks, are assigned in whole to single organizations or related groups of organizations, either by the Internet Corporation for Assigned Names and Numbers (ICANN), through the Internet Assigned Numbers Authority (IANA), or a regional Internet registry.
Special address blocks Address block (CIDR) First address Last address Number of addresses Usage Purpose ::/128 :: :: 1 Software Unspecified address
The subnet mask or CIDR notation determines how the IP address is divided into network and host parts. The term subnet mask is only used within IPv4. Both IP versions however use the CIDR concept and notation. In this, the IP address is followed by a slash and the number (in decimal) of bits used for the network part, also called the routing ...
For example, consider this IPv4 forwarding table (CIDR notation is used): 192.168.20.16/28 192.168.0.0/16 When the address 192.168.20.19 needs to be looked up, both entries in the forwarding table "match". That is, both entries contain the looked up address.
The Internet Assigned Numbers Authority (IANA) is a standards organization that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System (DNS), media types, and other Internet Protocol–related symbols and Internet numbers.
The default route in Internet Protocol Version 4 (IPv4) is designated as the zero address, 0.0.0.0 / 0 in CIDR notation. [2] Similarly, in IPv6, the default route is specified by :: / 0. The subnet mask is specified as / 0, which effectively specifies all networks and is the shortest match possible. A route lookup that does not match any other ...