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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: To indicate the size of a network or subnet for some routing protocols, such as OSPF. To indicate what IP addresses should be permitted or denied in access control lists ...
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. Shorter CIDR prefixes match more addresses ...
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 myIpAddress function has often been reported to give incorrect or unusable results, e.g. 127.0.0.1, the IP address of the localhost. [5] It may help to remove on the system's host file (e.g. /etc/hosts on Linux) any lines referring to the machine host-name, while the line 127.0.0.1 localhost can, and should, stay.
RFC 4271 corrected errors, clarified ambiguities and updated the specification with common industry practices. The major enhancement of BGP4 was the support for Classless Inter-Domain Routing (CIDR) and use of route aggregation to decrease the size of routing tables. RFC 4271 allows BGP4 to carry a wide range of IPv4 and IPv6