You have deployed a three-tier web application in a VPC with a CIDR block of 10.0.0.0/28. You
initially deploy two web servers, two application servers, two database servers and one NAT
instance for a total of seven EC2 instances. The web, application and database servers are
deployed across two availability zones (AZs).
You also deploy an ELB in front of the two web servers, and use Route53 for DNS. Web traffic
gradually increases in the first few days following the deployment, so you attempt to double the
number of instances in each tier of the application to handle the new load.
Unfortunately some of these new Instances fall to launch.
Which of the following could be the root cause? Choose 2 answers

A.
AWS reserves the first and the last private IP address in each subnet’s CIDR block so you do not
have enough addresses left to launch all of the new EC2 instances

B.
The Internet Gateway (IGW) of your VPC has scaled-up, adding more instances to handle the
traffic spike, reducing the number of available private IP addresses for new instance launches

C.
The ELB has scaled-up, adding more instances to handle the traffic spike, reducing the number of
available private IP addresses for new instance launches

D.
AWS reserves one IP address in each subnet’s CIDR block for Route53 so you do not have
enough addresses left to launch all of the new EC2 instances

E.
AWS reserves the first four and the last IP address in each subnet’s CIDR block so you do not
have enough addresses left to launch all of the new EC2 instances

Explanation:
http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html