Elastic IP addresses in AWS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

Elastic IP address behavior on instance stop and start

You have an EC2 instance with an Elastic IP (EIP) associated. What happens to the EIP when you stop and then start the instance?

A The Elastic IP is automatically disassociated and released when the instance stops.

B The Elastic IP is disassociated but not released; you must manually re-associate it after start.

C The Elastic IP remains associated with the instance after stop and start, preserving the public IP.

D The Elastic IP is released back to AWS and cannot be recovered after the instance stops.

💡 Hint

Think about how Elastic IPs are designed to provide a fixed public IP address.

Elastic IP addresses remain associated with the instance even if it is stopped and started. This allows the instance to keep the same public IP address.

❓ Architecture

intermediate

Using Elastic IPs for high availability

You want to design a highly available web service using two EC2 instances in different Availability Zones. How can Elastic IPs help in this design?

A Use a single Elastic IP and manually re-associate it to the healthy instance during failover.

B Assign an Elastic IP to each instance and use DNS failover to switch between them.

C Assign one Elastic IP to both instances simultaneously to share traffic.

D Elastic IPs cannot be used in high availability setups across Availability Zones.

💡 Hint

Consider how Elastic IPs can be moved between instances to maintain a fixed IP.

Elastic IPs can be reassigned from one instance to another in different Availability Zones to maintain a fixed IP during failover, enabling high availability.

❓ security

advanced

Security implications of Elastic IP address usage

Which of the following is a security best practice when using Elastic IP addresses in AWS?

A Regularly release unused Elastic IPs to avoid unnecessary charges and reduce attack surface.

B Assign Elastic IPs only to instances in private subnets to enhance security.

C Keep Elastic IPs allocated even when not in use to prevent others from using them.

D Use Elastic IPs to bypass security groups and network ACLs for easier access.

💡 Hint

Think about cost and security risks of unused public IPs.

Unused Elastic IPs incur charges and can increase the attack surface. Releasing them reduces costs and security risks.

❓ Configuration

advanced

Correct AWS CLI command to associate an Elastic IP

Which AWS CLI command correctly associates an Elastic IP address to an EC2 instance?

A aws ec2 associate-address --instance-id i-1234567890abcdef0 --allocation-id eipalloc-12345678 --public-ip 203.0.113.25

B aws ec2 associate-address --instance-id i-1234567890abcdef0 --public-ip 203.0.113.25

C aws ec2 associate-address --allocation-id eipalloc-12345678 --public-ip 203.0.113.25

D aws ec2 associate-address --instance-id i-1234567890abcdef0 --allocation-id eipalloc-12345678

💡 Hint

Check the required parameters for associating an Elastic IP to an instance.

The correct command uses the instance ID and the allocation ID of the Elastic IP to associate it.

✅ Best Practice

expert

Cost optimization with Elastic IP addresses

You have multiple Elastic IPs allocated but only some are associated with running instances. What is the best practice to optimize costs related to Elastic IPs?

A Keep all Elastic IPs allocated to avoid losing them, regardless of usage.

B Release Elastic IPs that are not associated with running instances to avoid hourly charges.

C Associate unused Elastic IPs to stopped instances to prevent charges.

D Convert Elastic IPs to standard public IPs to reduce costs.

💡 Hint

Consider AWS billing rules for Elastic IP addresses.

AWS charges for Elastic IPs that are allocated but not associated with a running instance. Releasing unused Elastic IPs avoids these charges.

Practice

1. What is the main purpose of an Elastic IP address in AWS?

easy

A. To provide a fixed public IP address that can be reassigned to different instances

B. To increase the storage capacity of an EC2 instance

C. To encrypt data in transit between AWS services

D. To automatically scale the number of EC2 instances

Solution

1. Step 1: Understand Elastic IP purpose

   An Elastic IP is a static public IP address designed to be associated with AWS resources like EC2 instances.

2. Step 2: Identify its main use

   It allows you to keep the same public IP even if you stop and start or replace the instance, ensuring consistent reachability.

3. Final Answer:

   To provide a fixed public IP address that can be reassigned to different instances -> Option A

4. Quick Check:

   Elastic IP = Fixed public IP for instances [OK]

Hint: Elastic IP means fixed public IP for your server [OK]

Common Mistakes:

• Confusing Elastic IP with storage or encryption
• Thinking Elastic IP auto-scales instances
• Assuming Elastic IP changes on instance restart

2. Which AWS CLI command correctly allocates a new Elastic IP address?

easy

A. aws ec2 create-elastic-ip

B. aws ec2 assign-elastic-ip

C. aws ec2 new-ip-address

D. aws ec2 allocate-address

Solution

1. Step 1: Recall AWS CLI syntax for Elastic IP allocation

   The correct command to allocate a new Elastic IP is 'aws ec2 allocate-address'.

2. Step 2: Verify other options

   Other options are invalid AWS CLI commands and will cause errors.

3. Final Answer:

   aws ec2 allocate-address -> Option D

4. Quick Check:

   Allocate Elastic IP = aws ec2 allocate-address [OK]

Hint: Allocate Elastic IP with 'allocate-address' command [OK]

Common Mistakes:

• Using non-existent commands like create-elastic-ip
• Confusing allocation with assignment commands
• Misspelling the command name

3. Consider this AWS CLI output after associating an Elastic IP to an instance:

{
  "AssociationId": "eipassoc-12345678",
  "PublicIp": "203.0.113.25",
  "InstanceId": "i-0abcd1234efgh5678"
}

What does this output indicate?

medium

A. The Elastic IP 203.0.113.25 is now linked to the instance i-0abcd1234efgh5678

B. The instance i-0abcd1234efgh5678 has been terminated

C. The Elastic IP has been released and is no longer usable

D. The instance has no public IP assigned

Solution

1. Step 1: Analyze the output fields

   The output shows an AssociationId, a PublicIp, and an InstanceId, indicating a link between the IP and instance.

2. Step 2: Interpret the meaning

   This means the Elastic IP 203.0.113.25 is assigned to the instance i-0abcd1234efgh5678.

3. Final Answer:

   The Elastic IP 203.0.113.25 is now linked to the instance i-0abcd1234efgh5678 -> Option A

4. Quick Check:

   AssociationId means IP linked to instance [OK]

Hint: AssociationId means IP linked to instance [OK]

Common Mistakes:

• Thinking the instance is terminated
• Assuming the IP is released
• Ignoring the AssociationId meaning

4. You tried to associate an Elastic IP to an instance but got an error: "AddressLimitExceeded". What is the likely cause?

medium

A. The instance does not have an internet gateway attached

B. You have reached the maximum number of Elastic IPs allowed in your AWS account

C. The Elastic IP is not in the same region as the instance

D. The instance is already terminated

Solution

1. Step 1: Understand the error message

   "AddressLimitExceeded" means you have hit the limit of Elastic IPs you can allocate in your AWS account.

2. Step 2: Check other options

   Instance termination or region mismatch cause different errors; internet gateway absence affects connectivity but not this error.

3. Final Answer:

   You have reached the maximum number of Elastic IPs allowed in your AWS account -> Option B

4. Quick Check:

   AddressLimitExceeded = Max Elastic IPs reached [OK]

Hint: AddressLimitExceeded means max Elastic IPs reached [OK]

Common Mistakes:

• Confusing region mismatch with limit error
• Assuming instance termination causes this error
• Thinking internet gateway absence triggers this error

5. You have an Elastic IP associated with an EC2 instance. You stop and start the instance. What happens to the Elastic IP and the instance's public IP?

hard

A. The Elastic IP remains but the instance loses its public IP until reassigned

B. The Elastic IP is released automatically and the instance gets a new public IP

C. The Elastic IP remains associated and the instance keeps the same public IP

D. The Elastic IP dissociates and you must manually re-associate it

Solution

1. Step 1: Recall Elastic IP behavior on instance stop/start

   Elastic IPs remain allocated and associated with the instance even if it is stopped and started.

2. Step 2: Understand public IP behavior

   Without Elastic IP, the public IP changes on stop/start, but with Elastic IP, the public IP stays the same.

3. Final Answer:

   The Elastic IP remains associated and the instance keeps the same public IP -> Option C

4. Quick Check:

   Elastic IP keeps public IP fixed on stop/start [OK]

Hint: Elastic IP keeps public IP fixed after stop/start [OK]

Common Mistakes:

• Thinking Elastic IP is released on stop/start
• Assuming public IP changes despite Elastic IP
• Believing manual reassociation is needed