

In this Quick Start, the sizing of CIDR blocks used in the subnets is
based on a typical deployment, where private subnets would have roughly
double the number of instances found in public subnets. However, during
deployment, you can use the CIDR block parameters to resize the CIDR
scopes to meet your architectural needs.

In the default subnet allocation, the VPC is divided into subnet types
and then further segmented per Availability Zone, as illustrated in
link:#figure1[Figure 1]. The Quick Start provides the following default
CIDR block sizes to maximize capacity:

[cols=",,",options="header",]
|===
|VPC |10.0.0.0/16 |
|Private subnets A |*10.0.0.0/17* |
| |Availability Zone 1 |10.0.0.0/19
| |Availability Zone 2 |10.0.32.0/19
| |Availability Zone 3 |10.0.64.0/19
| |Availability Zone 4 |10.0.96.0/19
|Public subnets |*10.0.128.0/18* |
| |Availability Zone 1 |10.0.128.0/20
| |Availability Zone 2 |10.0.144.0/20
| |Availability Zone 3 |10.0.160.0/20
| |Availability Zone 4 |10.0.176.0/20
|Private subnets B with dedicated custom network ACL |*10.0.192.0/19* |
| |Availability Zone 1 |10.0.192.0/21
| |Availability Zone 2 |10.0.200.0/21
| |Availability Zone 3 |10.0.208.0/21
| |Availability Zone 4 |10.0.216.0/21
|Spare subnet capacity |*10.0.224.0/19* |
| |Availability Zone 1 |10.0.224.0/21
| |Availability Zone 2 |10.0.232.0/21
| |Availability Zone 3 |10.0.240.0/21
| |Availability Zone 4 |10.0.248.0/21
|===

Alternatively, there may be situations where you would want to separate
the CIDR scopes by dividing the VPC into Availability Zones and then
into subnet types. The recommended CIDR blocks to maximize capacity for
this scenario are as follows:

[cols=",,",options="header",]
|===
|VPC |10.0.0.0/16 |
|Availability Zone 1 |*10.0.0.0/18* |
| |Private subnet A |10.0.0.0/19
| |Public subnet |10.0.32.0/20
| |Private subnet B |10.0.48.0/21
| |Spare subnet capacity |10.0.56.0/21
|Availibility Zone 2 |*10.0.64.0/18* |
| |Private subnet A |10.0.64.0/19
| |Public subnet |10.0.96.0/20
| |Private subnet B |10.0.112.0/21
| |Spare subnet capacity |10.0.120.0/21
|Availability Zone 3 |*10.0.128.0/18* |
| |Private subnet A |10.0.128.0/19
| |Public subnet |10.0.160.0/20
| |Private subnet B |10.0.176.0/21
| |Spare subnet capacity |10.0.184.0/21
|Availability Zone 4 |*10.0.192.0/18* |
| |Private subnet A |10.0.192.0/19
| |Public subnet |10.0.224.0/20
| |Private subnet B |10.0.240.0/21
| |Spare subnet capacity |10.0.248.0/21
|===

To customize the CIDR ranges for this scenario or to implement your own
segmentation strategy, you can configure the Quick Start parameters
described in link:#step-2.-launch-the-stack[step 2]. For more
information about VPC and subnet sizing, see the
https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html#VPC_Sizing[AWS documentation].