Elastic IP addresses in AWS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an Elastic IP address in AWS?

An Elastic IP address is a static, public IPv4 address designed for dynamic cloud computing. It lets you mask instance or availability zone failures by quickly remapping the address to another instance.

beginner

Why should you avoid leaving Elastic IP addresses unattached to running instances?

AWS charges for Elastic IP addresses that are allocated but not attached to a running instance to encourage efficient use of public IPs.

intermediate

How do Elastic IP addresses help with instance failure recovery?

You can quickly remap an Elastic IP address from a failed instance to a healthy one, minimizing downtime and maintaining the same public IP for clients.

intermediate

Can you associate multiple Elastic IP addresses with a single EC2 instance?

Yes, you can associate multiple Elastic IP addresses with a single instance if it has multiple network interfaces or secondary private IP addresses.

beginner

What is the difference between a public IP and an Elastic IP in AWS?

A public IP is assigned dynamically and changes when an instance stops or starts. An Elastic IP is static and remains the same until you release it, allowing consistent addressing.

What happens if you allocate an Elastic IP but do not associate it with a running instance?

A The Elastic IP is free to keep indefinitely

B The Elastic IP is automatically released

C The Elastic IP is assigned to a random instance

D You will be charged for the unused Elastic IP

AWS charges for Elastic IP addresses that are allocated but not attached to a running instance to encourage efficient use.

Which of the following best describes an Elastic IP address?

A An IPv6 address assigned to an instance

B A private IP address within a VPC

C A static public IPv4 address you can move between instances

D A dynamic IP address assigned at instance launch

Elastic IP addresses are static public IPv4 addresses that you can remap between instances.

How can Elastic IP addresses improve fault tolerance?

A By allowing quick remapping to a healthy instance after failure

B By automatically backing up instance data

C By increasing instance CPU capacity

D By encrypting network traffic

Elastic IPs let you quickly remap the address to another instance to reduce downtime.

Can you have more than one Elastic IP address associated with a single EC2 instance?

A Yes, if the instance has multiple network interfaces or secondary private IPs

B No, only one Elastic IP per instance is allowed

C Yes, but only in different regions

D No, Elastic IPs are shared across instances

Instances with multiple network interfaces or secondary private IPs can have multiple Elastic IPs.

What is the main difference between a public IP and an Elastic IP in AWS?

A Public IPs are static; Elastic IPs are dynamic

B Public IPs change on stop/start; Elastic IPs remain static until released

C Public IPs are only for private networks; Elastic IPs are public

D There is no difference

Public IPs assigned at launch can change when the instance stops and starts, while Elastic IPs remain fixed.

Explain what an Elastic IP address is and why it is useful in AWS.

Think about how you keep the same phone number even if you change phones.

Describe the cost implications of Elastic IP addresses and how to avoid unnecessary charges.

Unused resources in the cloud often cost money.

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