1. AWS VPC CIDR Rules:

• The allowed block size is between “/16” netmask and “/28” netmask
• You can create almost every CIDR that you want (like: 100.100.0.0/16, ….) But AWS Recommend follow RFC 1918 (Ref here: http://www.faqs.org/rfcs/rfc1918.html) as below

10.0.0.0        -   10.255.255.255  (10/8 prefix)
172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
192.168.0.0     -   192.168.255.255 (192.168/16 prefix)

• Example CIDR “15.15.0.0/16” still can be provisioned, but NOT RECCOMENDATION

• Some AWS services use the 172.17.0.0/16 CIDR range, avoid this range.

2. AWS VPC Subnet CIDR Rules:

• The allowed block size for subnet is between “/16” netmask and “/28” netmask
• The first four IP addresses and the last IP address in each subnet CIDR block are not available for your use, and they cannot be assigned to a resource. For example, in VPC (10.100.0.0/16) we have subnet (10.100.1.0/24), the following five IP addresses are reserved, CAN NOT be used.
  • 10.100.1.1: Reserved by AWS for the VPC router.
  • 10.100.1.2: Reserved by AWS. The IP address of the DNS server is the base of the VPC network range plus two
  • 10.100.1.3: Reserved by AWS for future use.
  • 10.100.1.255: Network broadcast address (AWS does not support broadcast)