CIDR Range is Class Less Inter Domain Routing, CIDR block is a collection of IP addresses that share the same network prefix and number of bits. The length of a prefix determines the size of CIDR blocks.
In a CIDR range, the first & last IP address will be reserved, one for Broadcast address and another one for Network Address when we make a below subnet only 2 IPs would be left for an actual use.
For example,
10.0.0.0/30 This CIDR has got 4 IP address totally
This range includes
the IPs from 10.0.0.0/30 to 10.0.0.3/30
10.0.0.0/30 à Reserved
10.0.0.1/30
10.0.0.2/30
10.0.0.3/30 à Reserved
10.0.0.5/30
10.0.0.6/30
10.0.0.7/30 à Reserved
When it comes to AWS, if we have VPC with the CIDR 10.0.0.0/24. The first four IP addresses and the last IP address in each subnet CIDR block are not available for you to use and cannot be assigned to an instance. For example, in a subnet with CIDR block 10.0.0.0/24,the following five IP addresses are reserved:
10.0.0.0: Network address.
10.0.0.1: Reserved
by AWS for the VPC router.
10.0.0.2: Reserved
by AWS for mapping to the Amazon-provided DNS.
10.0.0.3: Reserved
by AWS for future use.
10.0.0.255: Network
broadcast address.
Now let’s discuss about how to calculate the CIDR range. Below table shows the calculation of CIDR range based on the CIDR Mask Prefix.
Lower the CIDR mask prefix higher the IP count. Higher the CIDR mask prefix lower the IP count.
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