Network Access Control List (NACL) 

A Network Access Control List (NACL) is a crucial security component in Amazon Web Services (AWS) that controls traffic at the subnet level within a Virtual Private Cloud (VPC). Understanding how NACLs work is essential for building secure, scalable, and well-architected cloud networks.

This guide explains Network ACLs in a clear and structured manner, covering core concepts, real-world use cases, rule examples, comparisons, and best practices.

What is a Network Access Control List (NACL)?

A Network Access Control List (NACL) is a stateless firewall that controls inbound and outbound traffic for one or more subnets in an AWS VPC. It acts as a security layer that evaluates traffic based on defined rules before allowing or denying access.

Key Characteristics of NACL

• Operates at the subnet level
• Is stateless (responses must be explicitly allowed)
• Rules are evaluated in numerical order
• Supports both Allow and Deny rules
• Applies to all resources within a subnet

Why Use Network Access Control Lists?

NACLs provide an additional security boundary beyond Security Groups. They are particularly useful when you want to enforce network-wide rules across multiple resources.

Common Use Cases

• Blocking specific IP addresses or CIDR ranges
• Restricting traffic at the subnet level
• Adding an extra layer of defense for sensitive workloads
• Meeting compliance or regulatory requirements

How Network ACL Works

When traffic enters or leaves a subnet, AWS evaluates the NACL rules in ascending order (starting from the lowest rule number). The first rule that matches the traffic determines whether it is allowed or denied.

Traffic Flow Example

• Request enters the subnet
• NACL inbound rules are evaluated
• If allowed, traffic reaches the resource
• Response traffic must also be explicitly allowed by outbound rules

Inbound and Outbound Rules in NACL

Each NACL contains two sets of rules:

Inbound Rules

Control traffic entering the subnet.

Outbound Rules

Control traffic leaving the subnet.

Network ACL Rule Structure

Sample Network ACL Rules Example

Below is a practical example of inbound and outbound NACL rules:

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Inbound Rules:
Rule 100: Allow TCP port 80 from 0.0.0.0/0
Rule 110: Allow TCP port 443 from 0.0.0.0/0
Rule 120: Deny all traffic from 203.0.113.0/24
Rule *: Deny all

Outbound Rules:
Rule 100: Allow all traffic to 0.0.0.0/0

Explanation

• HTTP and HTTPS traffic is allowed from the internet
• A specific IP range is blocked
• All unspecified traffic is denied by default

NACL vs Security Group

Real-World Example Scenario

Consider an e-commerce application deployed in a public subnet:

• Allow HTTP/HTTPS traffic from the internet
• Block known malicious IP addresses
• Allow outbound access to payment gateways

Using a NACL ensures that malicious traffic never reaches the application layer.

Best Practices for Network ACL

• Use NACLs as an additional security layer, not a replacement for Security Groups
• Keep rule numbers organized with gaps (100, 110, 120)
• Explicitly allow ephemeral ports for return traffic
• Regularly audit and clean unused rules

Common Mistakes to Avoid

• Forgetting to allow outbound response traffic
• Using overly permissive CIDR ranges
• Misordering rules leading to unintended denies

Frequently Asked Questions (FAQs)

1. Is Network ACL stateful or stateless?

NACLs are stateless, meaning return traffic must be explicitly allowed by outbound rules.

2. Can one NACL be associated with multiple subnets?

Yes, a single Network ACL can be associated with multiple subnets within the same VPC.

3. What happens if no rule matches the traffic?

Traffic is denied by default if no rule explicitly allows it.

4. Should I use both NACL and Security Groups?

Yes, using both provides layered security and better protection.

5. Can NACL block specific IP addresses?

Yes, NACLs can explicitly deny traffic from specific IP ranges using CIDR blocks.

Network Access Control Lists (NACLs) play a vital role in securing AWS VPC networks. By controlling traffic at the subnet level, NACLs help protect resources from unauthorized access and malicious activity. When combined with Security Groups, they form a strong, layered security model.

Understanding how to design and manage NACL rules is essential for anyone working with AWS networking.