Core concept
Public subnet has route to IGW. Private via NAT Gateway (in public subnet). Isolated no internet route.
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How it works
Reserve 5 IPs per subnet for AWS use. Actual usable = CIDR - 5. Plan subnet sizes accordingly.
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Trade-offs + gotchas
ALB across public subnets (2+ AZs). EC2/RDS across private (2+ AZs). Multi-AZ = survives AZ failure.