Lesson 055 · AWS Learning Path

AWS 055: Public and private subnets

Anil Jalela · Published Aug 19, 2025 · 7 min read

The problem

A team uses Public and private subnets as a label or Console setting without proving the identity, scope, behavior, failure boundary, cost, or operational result. The configuration appears complete but the real requirement remains untested.

Final outcome

The learner will inspect, explain, test, and document Public and private subnets. The submission must connect the Console and CLI view to the same AWS control, state what the evidence proves, diagnose one likely failure, and make a requirement-based architecture decision.

Learning outcomes

You will be able to:

1. explain a subnet is an az-scoped address range;
2. explain public and private describe routing;
3. explain public ipv4 also matters;
4. explain aws reserves five addresses;
5. explain automatic public address assignment is a subnet setting;

1. distinguish successful configuration from successful workload behavior;
2. preserve redacted evidence and complete the stated cleanup or retention decision.

Mental model

requirement
   -> identity and permission
   -> account, Region, and resource scope
   -> configuration or request
   -> observable state
   -> workload result
   -> failure evidence
   -> cleanup or controlled retention

Never begin with a create button or a copied command. Start with the result that must be proven and the boundary that must remain protected.

Core facts

How to reason about it

Use one subnet per tier per Availability Zone rather than calling an entire VPC public or private. Public load balancers can occupy public subnets while application and database resources use private subnets. Routes define the path; security groups, network ACLs, identity policies, and application settings define whether access succeeds.

Use the following decision table as a starting point, then change the answer when the scenario changes.

Prerequisites, permissions, Region, and cost

Run this lesson as the approved non-root course identity. Begin with aws sts get-caller-identity, confirm the private account record, and set the fixed project Region before any regional query. IAM resources are global within an account, while STS endpoints and the services reached by an identity can be regional.

Use only the read or change actions required for this lesson. An AccessDenied result is evidence to analyse, not permission to switch to root or attach AdministratorAccess. Record the action, resource, principal type, request context, and smallest justified correction.

The listed cost tier is T0 - no resource creation. Free Tier eligibility and credits are account-specific. Before a mutating lab, identify every resource that can charge, estimate its duration, start a timer, and write the reverse cleanup order. A budget reports cost after billing data arrives and is not a real-time stop control.

AWS Management Console method

1. Open VPC Subnets and select one subnet.
2. Inspect its Availability Zone, CIDR, available addresses, public-IPv4 assignment attribute, route table, and network ACL.
3. Open the associated route table and classify the subnet from its actual gateway or NAT path, not its name tag.

For every step record the service page, selected account and Region, exact object, visible state, and why that state matters. A Console label or green status is not enough unless it is tied to the final workload outcome.

AWS CLI or API evidence

Join subnet attributes with their associated route tables before assigning a public or private label.

aws ec2 describe-subnets --query 'Subnets[].{Subnet:SubnetId,AZ:AvailabilityZone,CIDR:CidrBlock,AutoPublic:MapPublicIpOnLaunch,Free:AvailableIpAddressCount}' --output table
aws ec2 describe-route-tables --filters Name=association.subnet-id,Values=subnet-0123456789abcdef0

AutoPublic is not the route. Confirm a 0.0.0.0/0 route to an internet gateway and the resource public address separately.

Before running the command, replace every placeholder, explain each option, and decide whether the operation is read-only or mutating. Capture the exit code immediately. Redact account IDs, ARNs, public addresses, request identifiers, and personal data before sharing.

The CLI and Console are clients of AWS APIs. Matching state across them increases confidence, but neither substitutes for data-plane or application verification.

Practical work

Create p04-subnet-plan.md. Place the four approved /24 blocks across two Availability Zones using the zone names and zone IDs visible to this account. For every subnet state its tier, route intention, public-IPv4 assignment intention, NACL association, available-address calculation, and failure-domain purpose. Explain that a name containing public does not make a subnet public.

The evidence package must include:

• non-root principal type, account verified privately, and Region;
• exact intended and observed state;
• one Console observation and the matching CLI or API result;
• one successful result and one denied, failed, or counterexample result;
• what each result does not prove;
• cost state and elapsed lab time;
• cleanup evidence or a named retained-state owner and next lesson.

Verification standard

Use three levels of proof:

1. Control plane: the object or policy exists with the intended configuration.
2. Data plane or behavior: the request, packet, session, storage path, or application does what the requirement states.
3. Operations: monitoring, failure diagnosis, cost, ownership, and cleanup are known.

A control-plane response can precede final readiness. Use waiters or state polling where supported, then test the actual behavior. If a request times out, do not assume it failed. Inspect state and use documented idempotency before retrying a mutation.

Troubleshooting method

Keep the original symptom and timestamp. State one hypothesis, make one reversible change, repeat the original test, and record rollback. Never open a management port to the world, expose credentials, disable TLS verification, format an unknown disk, or add broad permissions as a generic fix.

Architecture and certification decisions

Professional-level questions provide competing valid features. Identify the requirement that decides between them: human or workload identity, same-account or cross-account access, regional or zonal scope, stateful or stateless filtering, durable or ephemeral data, latency, RTO/RPO, cost, or operational ownership.

Explain why the selected option fits and why each plausible alternative fails one stated requirement. Do not rely on feature memorization or reproduce protected certification questions.

Knowledge check

1. Does naming a subnet private make it private?

Expected direction: No. Inspect its routes, addressing, gateways, endpoints, and security controls.

1. How many usable IPv4 addresses are in an AWS /28 subnet?

Expected direction: Eleven because sixteen total addresses minus five AWS-reserved addresses remain.

1. Can one subnet span two Availability Zones?

Expected direction: No. Create a subnet for each required AZ.

1. Does a public subnet route automatically give an instance a public IPv4 address?

Expected direction: No. Public addressing is a separate requirement.

Cost, cleanup, and retained state

Retain only the state explicitly required by the next P04 lesson and record it in the private resource ledger.

Cleanup evidence includes the final state query, not only a successful delete response. Search related resources, other Regions used by the lab, retained storage, public IPv4 addresses, logging destinations, and service-managed dependencies. Schedule a later billing review because cost data can lag.

Completion gate

Pass when the practical artifact explains the real problem, matches Console and CLI evidence, answers every knowledge check, diagnoses one failure without broadening access, records cost, and proves cleanup or approved retention. The learner must defend one decision orally and name the requirement that would change it.

Official sources

• Subnets for your VPC
• Subnet CIDR blocks