Are Network ACLs an additional layer of security that act at the subnet level?

Network ACLs (Access Control Lists) indeed provide an additional layer of security that operates at the subnet level within an Amazon Virtual Private Cloud (VPC). They function as a stateless firewall for controlling inbound and outbound traffic to and from one or more subnets. Each subnet in a VPC can be associated with a Network ACL that specifies which IP addresses and protocols are allowed or denied access.

This capability is crucial for managing traffic flow at a broader level than security groups, which are more granular and operate at the instance level. Consequently, Network ACLs complement security groups by providing a way to enforce policies that apply to a wider range of resources, making them an essential part of a layered security approach.

While it's important to note that Network ACLs and security groups work together, Network ACLs do not override security groups; both types of security measures must permit traffic for communication to occur. This helps ensure that administrators have flexibility in defining combined rules for more complex security requirements.