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In a scenario where an EC2 instance is placed in a public subnet within a custom VPC, it is crucial to understand the conditions required for the instance to be accessible from the internet. For an instance to have internet accessibility in a public subnet, it typically requires an Elastic IP address or a public IP that allows it to be reached from outside the VPC.

When an EC2 instance is launched in a public subnet, although it has the potential to be internet accessible due to the associated route table directing traffic to an Internet Gateway, it does not automatically receive a public IP address unless explicitly configured to do so. Without a public IP or an Elastic IP, the instance will not be reachable from the internet, making it necessary to assign one for external accessibility.

This understanding is key to managing access to resources in AWS, as it allows architects to implement finer control over security and public exposure while still leveraging the benefits of cloud infrastructure. Thus, the assertion that the instance will not have internet accessibility unless an Elastic IP or public IP is applied aligns with the practical realities of AWS networking.