Amazon announced on August 29, 2017 that customers can now expand existing VPCs.
New AWS Feature
AWS customers can add up to 4 secondary CIDR blocks to an existing VPC. AWS cites two benefits of this new feature. First, customers can launch more resources (eg EC2 instances) in their VPCs on-demand. Second, customers don't have to over-allocate private IPv4 space when creating VPCs since you can add more in the future.
Engine Yard announced in May the ability to create Amazon VPCs in addition to the default VPC. A VPC is required to use new EC2 instance types like T2, C4, and M4 instances.
In most cases the default VPC per region is enough. But there are some cases where you want a new VPC.
One environment per VPC. If you want network isolation per environment, you can use a separate VPC for different environments like production and staging. You can also have 2 production environments on the same VPC while the rest use different VPCs. This is useful if you want to use the same database from 2 production environments.
VPC peering with another AWS account. Your Engine Yard environment running in a VPC can connect to a VPC from your second AWS account in the same region. This second AWS account can have EC2, RDS, ElastiCache, etc. A VPC peering connection between the two VPCs will work as long as they don't have overlapping CIDRs. You can choose the CIDR used on Engine Yard so this isn't a problem.
Back to the new AWS VPC feature. The ability to expand VPCs in theory will help Engine Yard customers. In practice, however, there is little need to increase the IPv4 space in the VPCs.
We create /24 subnets per availability zone. This results to 251 IP addresses available per availability zone (256 less 5 IP addresses reserved by AWS per subnet). In US East 1 (Northern Virginia), where there are 6 availability zones, you can boot up to 1,506 EC2 instances. Even in regions with 2 availability zones like London, Canada, Singapore, Seoul and Mumbai, you can boot up to 502 instances. This is plenty for our customers.
Do you have any questions or feedback about using VPC? Hit us up in the Comments section or open a Support ticket and our AWS-certified Support Team will help you.