In the beginning, the United States government and its contractors assigned IP number blocks by class, as follows:
CIDR is an efficient IP addressing scheme that reduces the size of routing tables and makes more IP numbers within the designated number block available for use as an IP address. CIDR notation specifies the starting number and size of a block and is expressed as an IP number block followed by a slash, followed by the decimal number of the leading bits of the routing prefix; for example: 192.168.0.0/16. The number to the right of the slash, in this case a 16, is the CIDR notation number block size. To count the quantity of contiguous numbers in the block, subtract the CIDR notation number block size from 32 and then raise 2 to that power – in other words, 2(32-CIDR) or in the case of our example, 216 = 65,536 or the size of an original Class B block.
The following IPv4 CIDR chart and is used by in the industry to see the number of IPv4 numbers contained within each CIDR length and the size of each block. The five Regional Internet Registries (RIRs) are responsible for managing and allocating IP number blocks and only deal with /24 and higher blocks. Addrex also only deals with /24 blocks and larger, up to /8 blocks.
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