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A subnet mask is a bitmask that encodes the prefix length associated with an IPv4 address or network in quad-dotted notation: 32 bits, starting with a number of 1-bits equal to the prefix length, ending with 0-bits, and encoded in four-part dotted-decimal format: 255.255.255.0. A subnet mask encodes the same information as a prefix length but ...
For example, a subnet mask of 255.255.255.0 (11111111.11111111.11111111.00000000 2) inverts to a wildcard mask of 0.0.0.255 (00000000.00000000.00000000.11111111 2). A wild card mask is a matching rule. [2] The rule for a wildcard mask is: 0 means that the equivalent bit must match; 1 means that the equivalent bit does not matter
For example, the subnet mask for a routing prefix that is composed of the most-significant 24 bits of an IPv4 address is written as 255.255.255.0. The modern standard form of specification of the network prefix is CIDR notation, used for both IPv4 and IPv6.
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. This is equivalent to the historically used subnet mask (in this case, 255.255.255.0).
In this example, the default gateway uses the IP address 192.168.4.1, which is resolved into a MAC address with ARP in the usual way. The destination IP address remains 192.168.12.3, but the next-hop MAC address is that of the gateway, rather than of the ultimate destination.
Some large / 8 blocks of IPv4 addresses, the former Class A network blocks, are assigned in whole to single organizations or related groups of organizations, either by the Internet Corporation for Assigned Names and Numbers (ICANN), through the Internet Assigned Numbers Authority (IANA), or a regional Internet registry.
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.
For example, the quad-dotted IP address in the illustration (172.16.254.1) represents the 32-bit decimal number 2886794753, which in hexadecimal format is 0xAC10FE01. CIDR notation combines the address with its routing prefix in a compact format, in which the address is followed by a slash character (/) and the count of leading consecutive 1 ...