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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.
A wildcard mask is a mask of bits that indicates which parts of an IP address are available for examination. In the Cisco IOS, [1] they are used in several places, for example:
IP addresses in dot-decimal notation are also presented in CIDR notation, in which the IP address is suffixed with a slash and a number, used to specify the length of the associated routing prefix. For example, 127.0.0.1/8 specifies that the IP address has an eight-bit routing prefix, and therefore the subnet mask 255.0.0.0 .
Special address blocks Address block Address range Number of addresses Scope Description 0.0.0.0/8 0.0.0.0–0.255.255.255 16 777 216: Software Current (local, "this") network [1]
In previous versions of the document, [19] [20] network numbers were 8-bit numbers rather than the 32-bit numbers used in IPv4. At that time, three networks were added that were not listed earlier: 42.rrr.rrr.rrr, 43.rrr.rrr.rrr, and 44.rrr.rrr.rrr. The relevant portion of RFC 790 is reproduced here with minor changes:
Since the introduction of CIDR, however, the assignment of an IP address to a network interface requires two parameters, the address and a subnet mask. Given an IPv4 source address, its associated subnet mask, and the destination address, a router can determine whether the destination is on a locally connected network or a remote network.
The CIDR notation for this group is 224.0.0.0 / 4. [1] The group includes the addresses from 224.0.0.0 to 239.255.255.255 . The address range is divided into blocks each assigned a specific purpose or behavior.
RFC 1918 name IP address range Number of addresses Largest CIDR block (subnet mask) Host ID size Mask bits Classful description [Note 1]; 24-bit block: 10.0.0.0 – 10.255.255.255: 16 777 216