IP "Supernetting": Classless Inter-Domain Routing (CIDR) Hierarchical Addressing and Notation
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When we first looked at IP addressing, we saw that IP addresses are designed to be divided into a network identifier and host identifier. Then, when subnets were introduced, we stole bits from the host ID to create a subnet ID, giving the IP address a total of three hierarchical levels. With VLSM, we further subnetted the subnets, taking more bits from the host ID to give us a multiple-level hierarchy with sub-subnets, sub-sub-subnets and so forth.
In a classless environment, we completely change how we look at IP addresses, by applying VLSM concepts not just to one network, but to the entire Internet. In essence, the Internet becomes just one giant network that is subnetted into a number of large blocks. Some of these large blocks are then broken down into smaller blocks, which can in turn be broken down further. This breaking down can occur multiple times, allowing us to split the pie of Internet addresses into slices of many different sizes, to suit the needs of organizations.
As the name implies, classless addressing completely eliminates the prior notions of classes. There are no more Class A, B, C blocks that are divided by the first few bits of the address. Instead, under CIDR, all Internet blocks can be of arbitrary size. Instead of having all networks use 8 (Class A), 16 (Class B) or 24 (Class C) bits for the network ID, we can have large networks with, say, 13 bits for the network ID (leaving 19 bits for the host ID), or very small ones that use 28 bits for the network ID (only 4 bits for the host ID). The size of the network is still based on the binary power of the number of host ID bits, of course.
Recall that when we used subnetting, we had a problem: subnetting could be done by taking any number of available host ID bits, so how would devices know where the line was between the subnet ID and host ID? The same problem occurs under CIDR. There are no classes, so we can't tell anything by looking at the first few bits of an IP address. Since addresses can have the dividing point between host ID and network ID occur anywhere, we need additional information in order to interpret IP addresses properly. Under CIDR, of course, this impacts not only addresses within an organization but in the entire Internet, since there are no classes and each network can be a different size.
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