Subnets: Public and Private

Subnets are the building blocks of a VPC. Every resource you deploy — an EC2 instance, an RDS database, a Lambda function with VPC access — must be placed in a subnet. Designing your subnet layout correctly is one of the most important decisions in any AWS architecture.

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What Is a Subnet?

A subnet is a logical division of a VPC's IP address range. Each subnet:

• Lives in exactly one Availability Zone (AZ)
• Has its own CIDR block that must be a subset of the VPC CIDR
• Is associated with exactly one route table
• Is associated with exactly one network ACL (NACL)

You cannot stretch a single subnet across multiple AZs. If you need resources in two AZs, you need at least two subnets.

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Public vs Private Subnets

The terms "public" and "private" are not formal AWS resource types — they describe how routing is configured.