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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 ...
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 ...
As shown in the example below, in order to calculate the directed broadcast address to transmit a packet to an entire IPv4 subnet using the private IP address space 172.16.0.0 / 12, which has the subnet mask 255.240.0.0, the broadcast address is calculated as 172.16.0.0 bitwise ORed with 0.15.255.255 = 172.31.255.255. Directed broadcasts always ...
A subnet mask encodes the same information as a prefix length but predates the advent of CIDR. In CIDR notation, the prefix bits are always contiguous. Subnet masks were allowed by RFC 950 [6] to specify non-contiguous bits until RFC 4632 [5]: Section 5.1 stated that the mask must be left contiguous. Given this constraint, a subnet mask and ...
IP multicast address range Description Routable 224.0.0.0 to 224.0.0.255 ... Routers must not forward these messages outside the subnet from which they originate.
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 prefix. For example, an IPv4 address and its subnet mask may be 192.0.2.1 and 255.255.255.0, respectively.
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. In this case, the longest prefix of the candidate routes is 192.168.20.16/28, since its subnet mask (/28) is longer than the other entry's mask (/16), making the route more specific.
IP packets originating from or addressed to a private IP address cannot be routed through the public Internet. Private addresses are often seen as enhancing network security for the internal network since use of private addresses internally makes it difficult for an external host to initiate a connection to an internal system.