Subnets and Availability Zones
A VPC spans an entire AWS Region. Subnets, however, live inside a single Availability Zone (AZ).
This design is intentional.
A subnet is a subdivision of your VPC CIDR block. For example:
- VPC:
10.0.0.0/16 - Subnet A:
10.0.1.0/24 - Subnet B:
10.0.2.0/24
Each subnet provides 256 IP addresses when using /24.
Why subnets matter architecturally:
- They segment workloads.
- They define failure domains (because they belong to a single AZ).
- They determine routing behavior.
High availability in AWS is achieved by distributing resources across multiple AZs. That means:
- At least two public subnets in different AZs.
- At least two private subnets in different AZs.
Production insight:
- Never deploy all resources in one AZ.
- Use small subnet blocks for workload isolation.
- Reserve space for future services (databases, internal services, containers).
Public vs Private Subnets:
A subnet becomes “public” when it has a route to an Internet Gateway. A subnet is “private” when it does not.
The subnet itself does not inherently know whether it is public or private. Routing defines that behavior.
Subnets and Availability Zones
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Why do subnets belong to a single Availability Zone?
In this section, I learned:
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