AWS Security Groups vs Network ACLs: Understanding Network Security in AWS

When learning AWS networking, creating a VPC is only the first step. We must also control who can access our resources and which network traffic is allowed. AWS provides two important security mechanisms for this: Security Groups and Network Access Control Lists (NACLs). Both help protect our cloud infrastructure, but they work at different levels.

Security Groups act like a personal firewall for an AWS resource, such as an EC2 instance. They control inbound and outbound traffic using rules based on ports, protocols, and traffic sources. Security Groups are stateful, meaning return traffic for an allowed connection is automatically permitted. For example, if we allow users to access a web server through HTTPS on port 443, the server can send responses without needing a separate rule for that return traffic.

Network ACLs work like security checkpoints for an entire subnet. They control traffic entering and leaving the subnet and support both allow and deny rules. Unlike Security Groups, NACLs are stateless, so inbound and outbound traffic must be handled separately. Their rules are evaluated in numerical order, and the first matching rule determines whether the traffic is allowed or denied.

Consider a simple web application hosted on AWS. Users access the web server, the web server communicates with the application server, and the application server connects to the database. Security Groups can restrict each resource to only the connections it needs, while NACLs can provide an additional layer of subnet-level protection. The key lesson is that secure cloud networking is not just about connecting resources; it is about allowing only the traffic that is necessary. Understanding these two mechanisms is an important step toward learning AWS security and DevSecOps.