Resolving Private IPv4 Exhaustion in AWS VPCs
Short answer
AC Associate a secondary IPv4 CIDR block with the VPC and create a new subnet for the VPC to resolve the lack of available private IPv4 addresses.
Question
Which combination of actions must the CloudOps engineer take to launch the instances? (Choose two.)
Key Takeaways
The question tests knowledge of VPC IP management limitations: you cannot change existing CIDRs, so you must add a secondary block and a new subnet to gain more IPs.
Learn how to fix EC2 launch failures caused by missing private IPv4 addresses by associating a secondary CIDR block and creating a new subnet in your AWS VPC.
Common mistake: Candidates often choose to modify the subnet CIDR (E), not realizing that AWS prohibits editing the CIDR range of an existing subnet, making this action impossible.
Expert Analysis
The core constraint is that the VPC lacks available private IPv4 addresses for new EC2 instances. This typically occurs when existing subnets are fully exhausted or when the VPC's primary CIDR block is too small to accommodate the desired scale. To resolve this, the engineer must expand the IP address space.
First, Associate a secondary IPv4 CIDR block with the VPC (Option A) is a standard AWS procedure. Since the primary IPv4 CIDR block cannot be changed or deleted after creation, adding a secondary CIDR block provides additional IP addresses without disrupting existing resources. This action increases the total pool of IPs available within the VPC.
Second, simply having a larger CIDR block is insufficient if the existing subnets do not utilize the new space. Subnet CIDR ranges cannot be modified after creation. Therefore, the engineer must Create a new subnet for the Vpc (Option C) using part of the newly associated secondary CIDR block. This new subnet will have its own range of available private IPv4 addresses, allowing the EC2 instances to launch successfully.
Other options are incorrect due to AWS limitations:
- Option B involves IPv6, which does not solve a shortage of private IPv4 addresses.
- Options D and E suggest modifying CIDR blocks, but AWS does not allow changing the CIDR of an existing VPC or subnet.
Thus, the combination of expanding the VPC's IP capacity (A) and provisioning a new subnet to use it (C) is the required solution.
Official Reference
https://docs.aws.amazon.com/vpc/latest/userguide/VPC_Subnet.html
This link opens the official vendor documentation in a new tab.
Exam Strategy
When troubleshooting resource exhaustion in AWS, always check if the service allows modification of existing configurations. For VPCs and Subnets, CIDR blocks are immutable once set. If you need more capacity, look for 'secondary' or 'additional' resource options rather than trying to resize existing ones. Memorize that Primary VPC/Subnet CIDRs cannot be changed.
Frequently Asked Questions
Can I change the CIDR block of an existing subnet?
Why can't I just add more IPs to the existing subnet?
Community Votes
75% of anonymous learners picked answer AC. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 2 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 2
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testerhelper 👍 1• Primary IPv4 CIDR: The initial IPv4 CIDR block assigned to a VPC cannot be changed or deleted. • Subnet CIDR Ranges: Existing subnet IPv4 address ranges cannot be directly edited
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Bad_Mat 👍 1The correct combination of actions is:A. Associate a secondary IPv4 CIDR block with the VPC.C. Create a new subnet for the VPC.
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