understanding AWS CIDR

In AWS, Classless Inter-Domain Routing (CIDR) blocks play a crucial role in understanding VPC.

  • CIDR blocks are used to specify a range of IP addresses in IPv4 or IPv6 format.
  • The general format for CIDR blocks is x.y.z.t/p, where x, y, z, and t are numbers from 0 to 255 representing 8-bit binary numbers.
  • In AWS, p is a number from 16 to 28, indicating the number of bits inherited from the given IP address.
  • Each CIDR block in AWS prohibits 5 possible IP addresses: network address, VPC router, DNS server, reserved for future use, and network broadcast.
  • AWS permits a numeric value of p up to /28 because it needs 5 IP addresses, and for p=30, there would be only 4 available IP addresses.
  • The number of possible IP addresses in a CIDR block can be calculated using the formula: NumberOfPossibleIPs = 2^(32-p) – 5.
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