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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 ...
Subnet Used for link-local addresses [5] between two hosts on a single link when no IP address is otherwise specified, such as would have normally been retrieved from a DHCP server 172.16.0.0/12 172.16.0.0–172.31.255.255 1 048 576: Private network Used for local communications within a private network [3] 192.0.0.0/24 192.0.0.0–192.0.0.255 256
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.
A subnetwork, or subnet, is a logical subdivision of an IP network. [ 1 ] : 1, 16 The practice of dividing a network into two or more networks is called subnetting . Computers that belong to the same subnet are addressed with an identical group of its most-significant bits of their IP addresses .
IP was the connectionless datagram service in the original Transmission Control Program introduced by Vint Cerf and Bob Kahn in 1974, which was complemented by a connection-oriented service that became the basis for the Transmission Control Protocol (TCP). The Internet protocol suite is therefore often referred to as TCP/IP.
2. Subnet mask (The /12 in the IP address in this case means only the left-most 12 bits are 1s, as shown here. This reserves the left 12 bits for the network address (prefix) and the right 32 - 12 = 20 bits for the host address (suffix).) 11111111.11110000.00000000.00000000: 255.240.0.0 3. Bit complement (bitwise NOT) of the subnet mask
This means that 256 /8 address blocks fit into the entire IPv4 space. As IPv4 address exhaustion has advanced to its final stages, some organizations, such as Stanford University , formerly using 36.0.0.0 / 8 , have returned their allocated blocks (in this case to APNIC ) to assist in the delay of the exhaustion date.
The utility is a command-line interface tool and is also used in the system startup scripts of many operating systems. It has features for configuring, controlling, and querying TCP/IP network interface parameters. Ifconfig originally appeared in 4.2BSD as part of the BSD TCP/IP suite.