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CIDR notation is a compact representation of an IP address and its associated network mask. The notation was invented by Phil Karn in the 1980s. [9] [10] CIDR notation specifies an IP address, a slash ('/') character, and a decimal number. The decimal number is the count of consecutive leading 1-bits (from left to right) in the network mask ...
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:
Each route is summarized before being advertised to other districts. Each router now only recognizes its own subnet and the other 49 summarized routes. The determination of the summary route on a router involves the recognition of the number of highest-order bits that match all addresses. The summary route is calculated as follows.
The subnet mask or CIDR notation determines how the IP address is divided into network and host parts. 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 ...
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
This is a list of the IP protocol numbers found in the field Protocol of the IPv4 header and the Next Header field of the IPv6 header. It is an identifier for the encapsulated protocol and determines the layout of the data that immediately follows the header. Both fields are eight bits wide.
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 ...
The number of addresses usable for addressing specific hosts in each network is always 2 N - 2, where N is the number of rest field bits, and the subtraction of 2 adjusts for the use of the all-bits-zero host value to represent the network address and the all-bits-one host value for use as a broadcast address. Thus, for a Class C address with 8 ...