CIDR: Classless Interdomain Routing
CIDR: Classless Interdomain Routing 관련
One of the measures to handle this shortage of addresses was to abandon classful addressing in 1993 and switch to another approach called CIDR – Classless Interdomain Routing. This approach is still used today.
CIDR allows for flexibility when choosing the network ID and the host ID. It lets network administrators create subnets of precisely the right size, rather than being limited to Classes A, B, or C.
Let's start with a simple example. In CIDR notation, we add a suffix indicating how many bits are used for the network portion:
(4) 200.8.3.1/16
This slash notation specifies how many bits describe the network ID. In example (4) above, the first 16 bits (or 2 bytes) are used for the network ID. So, in this case, 200.8 is the network identifier, and 3.1 is the host identifier. The fact that 200.8 is the network ID means that all addresses from 200.8.0.0 through 200.8.255.255 are in this network.
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BriefVid)Consider these additional addresses:
(5) 200.2.13.5
(6) 200.8.21.6
Given this address prefix of 16 bits, or 2 bytes, which of these addresses belong to the same network as example (4) (200.8.3.1/16)?
The first address (5) (200.2.13.5) does not belong to this network, as its first 16 bits – 200.2, are different from the first 16 bits of the example address.
The second address (6) (200.8.21.6) does belong to the same network as that of the example address.
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BriefVid)Real-world Example
In practice, an ISP might receive a large block like 104.16.0.0/12 from the RIR. This gives them control of all addresses from 104.16.0.0 to 104.31.255.255. The ISP can then allocate smaller subnets to customers, such as giving a small business a /24 subnet with 256 addresses, or a larger company a /20 subnet with 4,096 addresses.