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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 (ACLs). A wildcard mask can be thought of as an inverted subnet mask.
A special definition exists for the IP address 255.255.255.255. It is the broadcast address of the zero network or 0.0.0.0, which in Internet Protocol standards stands for this network, i.e. the local network. Transmission to this address is limited by definition, in that it is never forwarded by the routers connecting the local network to ...
In previous versions of the document, [19] [20] network numbers were 8-bit numbers rather than the 32-bit numbers used in IPv4. At that time, three networks were added that were not listed earlier: 42.rrr.rrr.rrr, 43.rrr.rrr.rrr, and 44.rrr.rrr.rrr. The relevant portion of RFC 790 is reproduced here with minor changes:
In computer science, a mask or bitmask is data that is used for bitwise operations, particularly in a bit field.Using a mask, multiple bits in a byte, nibble, word, etc. can be set either on or off, or inverted from on to off (or vice versa) in a single bitwise operation.
255 is a special number in some tasks having to do with computing. This is the maximum value representable by an eight-digit binary number, and therefore the maximum representable by an unsigned 8-bit byte (the most common size of byte, also called an octet), the smallest common variable size used in high level programming languages (bit being smaller, but rarely used for value storage).
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
Using octets with all eight bits set, the representation of the highest-numbered IPv4 address is 255.255.255.255. An IPv6 address consists of sixteen octets, displayed in hexadecimal representation (two hexits per octet), using a colon character (:) after each pair of octets (16 bits are also known as hextet ) for readability, such as 2001:0db8 ...
224.0.0.0–255.255.255.255 The advantage of this system is that the network prefix could be determined for any IP address without any further information. The disadvantage is that networks were usually too big or too small for most organizations to use, because only three sizes were available.