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Getting Started with Networking: Understand Key Properties and Limitations of AWS Subnets

By: Author Alex Lim

Posted on

Categories Networking

Learn the truth about AWS subnets – spanning availability zones, subnet sizes, route table and VPC associations. Master subnets for the AWS Certified Cloud Practitioner exam.

Table of Contents

Question

Which statement about subnets is true?

A. Subnets cannot span multiple Availability Zones.
B. Subnets can be associated with multiple route tables.
C. Subnets can be associated with multiple VPCs.
D. You can reconfigure the size of your subnet.

Answer

A. Subnets cannot span multiple Availability Zones.

Explanation

Subnets cannot span multiple Availability Zones. When you create a subnet, you cannot reconfigure the size or associate it with multiple route tables. You also cannot assign it to a VPC other than the one in which it was created.

In Amazon Web Services (AWS), a subnet is a range of IP addresses within a Virtual Private Cloud (VPC). Each subnet must reside entirely within one Availability Zone and cannot span across multiple zones.

Availability Zones are distinct locations engineered to be isolated from failures in other Availability Zones. By launching instances in separate AZs, you can protect your applications from the failure of a single location. Subnets enable you to group instances based on your security and operational needs within a single AZ.

Some other key properties of subnets:

  • A subnet can only be associated with one route table at a time, not multiple route tables (eliminating option B). The subnet’s traffic is routed according to the associated route table.
  • A subnet belongs to one specific VPC and cannot be reassigned to a different VPC (eliminating option C). Once created, you cannot change which VPC a subnet is part of.
  • The size of a subnet’s CIDR block is fixed at creation time and cannot be changed later (eliminating option D). Make sure to choose a subnet size that allows for future growth.

In summary, the statement that subnets cannot span multiple Availability Zones is the only true statement among the provided options. Understanding the properties and limitations of subnets is important for properly designing and securing your cloud architecture on AWS.

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