AWS supported CIDR block range

The allowed block size is between a /16 netmask (65,536 IP addresses) and /28 netmask (16 IP addresses).

VPC Limits

Resource Default limit
VPCs per Region 5
Subnets per VPC 200
IPv4 CIDR blocks per VPC 5
IPv6 CIDR blocks per VPC 5

Private vs Public IP (IPV4) Fundamental Differences

Public IP:

  1. Public IP means the machine can be identified on the internet (WWW)
  2. Must be unique across the whole web (not two machines can have the same public IP).
  3. Can be geo-located easily

Private IP:

  1. Private IP means the machine can only be identified on a private network only
  2. The IP must be unique across the private network
  3. BUT two different private networks (two companies) can have the same IPs.
  4. Machines connect to WWW using an internet gateway (a proxy)
  5. Only a specified range of IPs can be used as private IP

Note: When you stop/start your instance, the IP address will change. If you reboot the instance, it will keep the same IP addresses. Unfortunately, it is not possible for us to reassign the address to your instance as that address would have been released back into the pool used by other EC2 instances.

Why use a custom AMI?

Using a custom built AMI can provide the following advantages:

  1. Pre-installed packages needed
  2. Faster boot time (no need for ec2 user data at boot time)