AWS Security Groups vs Network ACLs: Understanding Network Security in AWS
Learn the key differences between AWS Security Groups and Network ACLs, how they control network traffic, and how to use them together to build a more secure AWS environment.
AWS Security Groups vs Network ACLs: Understanding Network Security in AWS
When I started learning AWS, networking and security were two areas that required the most hands-on practice. One of the most important concepts I learned was the difference between Security Groups and Network Access Control Lists (NACLs).
Both are used to control network traffic in AWS, but they work at different levels and have different behaviors.
What is a Security Group?
A Security Group acts as a virtual firewall for an AWS resource such as an EC2 instance.
Security Groups control traffic at the instance level. They contain rules that determine which inbound and outbound traffic is allowed.
For example, an EC2 instance running a web server may need:
- HTTP (port 80) for web traffic
- HTTPS (port 443) for secure web traffic
- SSH (port 22) for administrative access
A Security Group rule could allow SSH only from a trusted IP address instead of allowing SSH access from anywhere.
This is an important security practice because unnecessary public access can increase the attack surface.
What is a Network ACL?
A Network ACL, or NACL, is a network-level security control associated with a subnet.
Unlike Security Groups, NACLs can control traffic entering and leaving a subnet.
NACL rules can allow or deny traffic based on information such as:
- Protocol
- Port range
- Source IP address
- Destination IP address
This makes NACLs useful when we need an additional layer of network-level control.
Security Groups vs NACLs
The easiest way to remember the difference is:
Security Group → Resource/instance level
NACL → Subnet level
There are several other important differences.
Security Groups
- Associated with network interfaces/resources
- Stateful
- Support allow rules
- Return traffic is automatically allowed for an established connection
- Commonly used to control access to individual resources
Network ACLs
- Associated with subnets
- Stateless
- Support both allow and deny rules
- Inbound and outbound traffic are evaluated separately
- Useful for controlling traffic at the subnet boundary
A Simple Example
Imagine a VPC containing a public subnet with an EC2 web server.
A simplified security design could look like this:
Internet
↓
Network ACL
↓
Subnet
↓
Security Group
↓
EC2 Instance
The NACL provides a subnet-level layer of control, while the Security Group provides a more specific layer of control for the EC2 instance.
Using both can provide defense in depth.
Why Should We Avoid Opening Everything?
During practice, it can be tempting to allow traffic from:
1
0.0.0.0/0
for every required port.
However, this means the resource is potentially accessible from any IPv4 address.
For example, opening SSH port 22 to the entire internet is generally not a good security practice when access can instead be restricted to a trusted source.
A better approach is to allow only the traffic that is actually required.
Security as a Layered Approach
AWS security should not depend on a single control.
A secure architecture can combine multiple layers such as:
- IAM for identity and permissions
- Security Groups for resource-level network access
- NACLs for subnet-level traffic control
- Encryption for protecting data
- CloudTrail for auditing API activity
- CloudWatch for monitoring
- AWS security services for additional protection
This approach is often described as defense in depth.
What I Learned
One of the biggest lessons from practicing AWS networking is that security is not simply about blocking traffic.
It is about understanding:
Who needs access?
What access do they need?
From where should they access it?
Which ports and protocols are required?
By asking these questions before creating firewall rules, we can build more controlled and predictable cloud environments.
Practical Checklist
Before deploying an AWS workload, I try to check:
- Is public access actually required?
- Are unnecessary ports closed?
- Is SSH restricted to a trusted source?
- Are Security Group rules as specific as possible?
- Are NACL rules configured intentionally?
- Are IAM permissions following least privilege?
- Are logs and monitoring enabled where appropriate?
- Are unused resources and access rules removed?
Conclusion
Security Groups and Network ACLs are fundamental building blocks of AWS network security.
The key difference is simple:
Security Groups protect resources, while NACLs control traffic at the subnet level.
Understanding how these controls work together is an important step toward designing secure AWS infrastructure.
For anyone learning AWS or starting a DevOps journey, I recommend practicing these concepts in a test VPC and observing how different network rules affect connectivity.
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