Explanation:
CIDR (Classless Inter-Domain Routing) notation is a compact representation of an IP address and its associated routing prefix. The notation is constructed from an IP address, a slash ('/') character, and a decimal number. The number is the count of leading 1 bits in the subnet mask. Larger values here indicate smaller networks. CIDR was developed as an alternative to traditional subnetting. The idea is that you can add a specification in the IP address itself as to the number of significant bits that make up the routing or networking portion.
IPv4 CIDR blocks interpretation :
IPv4 : a.b.c.d
Bits : 8.8.8.8 --> 32 bits
CIDR block Notation : a.b.c.d/prefix where prefix is the number of IP address the network can have.
Example : 192.168.100.0/22
IPs calculation for the above CIDR block will be [192(8bits).168(8bits).100(8bits).0(8bits)] = 2^32-22 = 2^10 = 1,024
where 32 -> 8 * 4 bits , 22 -> prefix
Total number of IPs for 192.168.100.0/22 CIDR block will be 1,024
192.168.100.0 -> 255
192.168.101.0 -> 255
192.168.102.0 -> 255
192.168.103.0 -> 255
where 0 -> 255 = 256 IPs
Total IPs = 256 * 4 = 1,024
By default, AWS uses 5 IPs out of these 256 IPs, So the number of IPs released will be 256 - 5 = 251
IPs used by AWS:
0 -> Network Address
1 -> Amazon VPC Routing
2 -> Domain Name System (DNS)
3 -> Future use by AWS
255 -> Broadcasting
In AWS, a subnet can have 1 primary CIDR block and 4 secondary CIDR blocks
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