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An IP address is part of a CIDR block and is said to match the CIDR prefix if the initial n bits of the address and the CIDR prefix are the same. An IPv4 address is 32 bits so an n -bit CIDR prefix leaves 32 − n bits unmatched, meaning that 2 32− n IPv4 addresses match a given n -bit CIDR prefix.
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.
In the above example, the subnet mask consists of 26 bits, making it 255.255.255.192, leaving 6 bits for the host identifier. This allows for 62 host combinations (2 6 −2). In general, the number of available hosts on a subnet is 2 h −2, where h is the number of bits used for the host portion of the address.
Shared address space [4] for communications between a service provider and its subscribers when using a carrier-grade NAT 127.0.0.0/8 127.0.0.0–127.255.255.255
CIDR notation can also be used to designate how much of the address should be treated as a routing prefix. For example, 192.0.2.1 / 24 indicates that 24 significant bits of the address are the prefix, with the remaining 8 bits used for host addressing.
The need to record routes to large numbers of devices using limited storage space represents a major challenge in routing table construction. In the Internet, the currently dominant address aggregation technology is a bitwise prefix matching scheme called Classless Inter-Domain Routing (CIDR).
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 ...