Classless inter-domain routing (CIDR)
Classless inter-domain routing or CIDR is a set of internet protocol standards that helps in creating unique identifiers for various networks and devices.
It consists of two groups of numbers, also known as the groups of bits. The most important of these groups is the network address that is used to identify the network or the subnet. A host identifier is used to determine which host will receive the incoming information packets. CIDR also allows the blocking of IP addresses to be allocated to the internet service providers.
CIDR Blocks
CIDR can group the block of addresses into a single routing network. CIDR block shares the first part of the bit sequence and the blocks are identified using the same decimal-dot CIDR notation system which was used for IPV4. Nowadays IPV6 is used for CIDR. Addresses having the same number of bits and prefixes are put in the same block.
Internet Assigned Numbers Authority handles the assignment of CIDR blocks. It also issues a large number of IP addresses to regional internet registries. The size of the block depends on each individual user.
Subnet Masks
Once the blocks of addresses are assigned to the end-users, they can be further divided through subnetting.
Then the subnet becomes the most significant portion of the host identifier. The subnet identifiers are assigned according to the network’s subnet masks. They are generally expressed using a decimal-dot expression, similar to the one used for IP addresses.
To create a specific subnet mask, a portion of the host identifier is designated, and to create larger subnets, more bits are moved from the host identifier to the subnet mask. The final subnet has binary notation with all ones.
Reference
Classless inter-domain routing (CIDR)