Public vs private subnets

beginner

Scenario & brief

A basic three-tier app is wired up wrong: the load balancer isn't reachable, while the app servers and database are exposed to the public internet. That's backwards and dangerous.

Get the VPC layout right:

the ALB should be internet-facing (users must reach it),
the app servers and database belong in private subnets,
the database must not be reachable from the internet, and
private subnets need a NAT gateway for outbound updates.

1,000 rps peakp99 ≤ 200ms99.9% availdurability: standardbudget $200/mo

Load Balancer

Networking

Subnet placement

Allowed ingress (security group)

System health

Erupting · SLA breach

/ 100

Score

SLA not met yet

Monthly cost

$22

Budget $200/mo · within budget

Metrics

Security posture0

Connectivity0

Requirements met0

Requirements

Load balancer is internet-facing — not reachable (need internet-facing)
App servers in a private subnet — public subnet (need private subnet)
Database in a private subnet — public subnet (need private subnet)
Database not reachable from the internet — internet-reachable (need not internet-reachable)
Private subnets egress via NAT — no NAT (need private egress via NAT)

Advisor

Load balancer is internet-facing: Place it in a public subnet with internet ingress so users can reach it.
App servers in a private subnet: Move it to the private subnet.
Database in a private subnet: Move it to the private subnet.
Database not reachable from the internet: Put it in a private subnet, drop the public IP, and don't allow ingress from the internet.
Private subnets egress via NAT: Add a NAT gateway so private-subnet resources can reach the internet for updates without being exposed.

Discussion

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For learning purposes only. Costs and capacities are illustrative, not live AWS prices. Not affiliated with or endorsed by Amazon Web Services.