A legacy application with a license is attached to a single MAC address. An EC2 instance can receive a new MAC address while launching new instances. How can you ensure that your EC2 instances can maintain a single MAC address for licensing? Choose the correct option.

Answer options:

A.Create an ENI and assign it to the EC2 instance. The ENI will have a static MAC address and can be detached and reattached to a new instance if the current instance becomes unavailable.
B.Private subnets have static MAC addresses. Launch the EC2 instance in a private subnet and, if required, use a NAT to serve data over the internet.
C.Configure a manual MAC address for each EC2 instance and report that to the licensing company.
D.AWS cannot have a fixed MAC address; the best solution is to create a dedicated VPN/VGW gateway to serve data from the legacy application.

Answer – A
Tip: Whenever a question has a scenario where you need to use a fixed MAC address for EC2 instances, always think about using Elastic Network Interface (ENI).
If a static MAC address is assigned to an ENI, it remains unchanged. As long as the EC2 has that ENI, its MAC address will not change.
Option A is CORRECT because, as mentioned above, as ENI with static MAC address can be assigned to the EC2 instance. If the instance becomes unavailable or needs to be replaced, the ENI can be detached and re-attached to another EC2 while maintaining the same MAC address.
Option B is incorrect because subnets have CIDR, not static MAC addresses.
Option C is incorrect because if the EC2 instance fails or becomes unavailable, its MAC address cannot be reused with another EC2 instance.
Option D is incorrect because you can avail of ENI in order to have a static MAC address for the EC2 instances.
More information on ENI on AWS Documentation:
Create a Low Budget High Availability Solution
If one of your instances serving a particular function fails, its network interface can be attached to a replacement or hot standby instance pre-configured for the same role in order to recover the service rapidly. For example, you can use a network interface as your primary or secondary network interface to a critical service such as a database instance or a NAT instance. If the instance fails, you (or more likely, the code running on your behalf) can attach the network interface to a hot standby instance. Because the interface maintains its private IP addresses, Elastic IP addresses, and MAC address, network traffic begins flowing to the standby instance as soon as you attach the network interface to the replacement instance. Users experience a brief loss of connectivity between the time the instance fails and when the network interface is attached to the standby instance, but no changes to the VPC route table or your DNS server are required.
Best Practices for Configuring Network Interfaces
For more information on elastic network interfaces, please visit the below URL-
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html