Lesson 074 · AWS Learning Path

AWS 074: Elastic IP and network interfaces

Anil Jalela · Published Sep 7, 2025 · 8 min read

The problem

A team uses Elastic IP and network interfaces 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 Elastic IP and network interfaces. 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 an eni is a virtual network card;
2. explain the primary eni has lifecycle limits;
3. explain an elastic ip is static public ipv4;
4. explain public ipv4 addresses are billable;
5. explain private addresses belong to the eni;

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

Separate instance lifecycle from network identity. An ENI can hold stable private addresses and security groups, while an Elastic IP supplies a stable public IPv4 mapping. Managed load balancers, DNS failover, and Auto Scaling are often better recovery mechanisms than manually moving interfaces.

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 EC2 Network Interfaces and inspect the interface subnet, Availability Zone, addresses, security groups, attachment, and source/destination check.
2. Open Elastic IP addresses and inspect the current association and network-interface mapping.
3. Before moving an interface or address, confirm same-AZ restrictions and whether a load balancer or DNS design is safer.

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

Inspect interface ownership, addresses, groups, and source/destination checking.

aws ec2 describe-network-interfaces --network-interface-ids eni-0123456789abcdef0 --query 'NetworkInterfaces[0].{AZ:AvailabilityZone,Subnet:SubnetId,Private:PrivateIpAddresses,Groups:Groups,Check:SourceDestCheck,Attachment:Attachment}'

Confirm whether the interface is primary or secondary before planning detachment. Its AZ restricts which instances can attach it.

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

Inspect the stopped nw-p04-web-1 after AWS 073. Create p04-interface-map.md with primary ENI, subnet, AZ, private IPv4, current public-IPv4 field, the previously recorded temporary public IPv4, security groups, attachment index, source/destination check, and lifecycle ownership. Explain why the automatically assigned public IPv4 was released at stop while the private IPv4 and primary ENI remain. Compare a secondary ENI and Elastic IP recovery pattern with load balancer and DNS alternatives. Do not start the instance or allocate an Elastic IP in this lesson.

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. Can a secondary ENI move to an instance in another AZ?

Expected direction: No. The ENI is bound to one subnet and AZ.

1. Does an Elastic IP appear inside the guest operating system?

Expected direction: Normally no. The guest sees the private address and the internet gateway performs the public mapping.

1. Why disable source/destination checking on a NAT instance?

Expected direction: The instance forwards packets whose source or destination is another host.

1. Are unused public IPv4 addresses the only charged public IPv4 addresses?

Expected direction: No. AWS current pricing also charges for public IPv4 addresses in use.

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

• Elastic network interfaces
• Elastic IP addresses