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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:
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.
3. Bit complement (bitwise NOT) of the subnet mask 00000000.00001111.11111111.11111111: 0.15.255.255 4. Broadcast address (bitwise OR of 1. Network IP address and 3. Bit complement of the subnet mask. This makes the broadcast address the largest possible IP address (and host address, since the host address portion is all 1s) for any given ...
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
Routing tables are also a key aspect of certain security operations, such as unicast reverse path forwarding (uRPF). [2] In this technique, which has several variants, the router also looks up, in the routing table, the source address of the packet. If there exists no route back to the source address, the packet is assumed to be malformed or ...
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 ...
For example, an IPv4 address and its subnet mask may be 192.0.2.1 and 255.255.255.0, respectively. The CIDR notation for the same IP address and subnet is 192.0.2.1 / 24, because the first 24 bits of the IP address indicate the network and subnet.
Internet Group Management Protocol (IGMP) version 3 [22]: §4.2.14 No 224.0.0.102 Hot Standby Router Protocol version 2 (HSRPv2) / Gateway Load Balancing Protocol (GLBP) No 224.0.0.107 Precision Time Protocol (PTP) version 2 peer delay measurement messaging No 224.0.0.251 Multicast DNS (mDNS) address No 224.0.0.252