A subdivision of a VPC's IP address range, typically used to separate public-facing resources from private ones and to spread resources across multiple availability zones for resilience.
Subnets carve a VPC's CIDR (say 10.0.0.0/16) into smaller ranges (10.0.1.0/24, 10.0.2.0/24, etc.). The most common pattern is one public subnet and one private subnet per Availability Zone. Public subnets host load balancers and bastion hosts (internet-routable), and private subnets host application servers and databases (internet-isolated). Subnets also influence service placement: RDS in private subnets, ELBs in public subnets, etc.
Designing 6 subnets across 3 AZs (3 public, 3 private) for a multi-AZ app, which survives any single AZ failure.
Multi-AZ subnet planning is the foundation of cloud high availability. Single-AZ designs fail when (not if) that AZ has an outage.
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