Learn what a Network ACL (NACL) in AWS is, how inbound and outbound rules work, and how it differs from Security Groups with real-world examples.
Introduction
A Network ACL (NACL) is the subnet-level firewall in AWS.
If Security Groups protect individual instances, NACLs protect the entire subnet. Understanding both is critical for designing secure cloud systems.
In this guide, you’ll learn:
- What a NACL is
- How inbound and outbound rules work
- How it differs from Security Groups
- Real-world architecture usage
- Best practices
What is a Network ACL (NACL) in AWS?
A Network Access Control List (NACL) is a stateless firewall that controls traffic in and out of a subnet.
In Amazon Web Services, NACLs act as an additional layer of security beyond Security Groups.
Simple Definition
A NACL is a firewall at the subnet level that allows or denies traffic based on rules.
How NACL Works
1. Inbound Rules
- Control incoming traffic to subnet
2. Outbound Rules
- Control outgoing traffic from subnet
Stateless Nature (CRITICAL CONCEPT)
NACL is stateless, meaning:
You must explicitly allow:
- Incoming traffic
- AND outgoing response
Example:
If you allow:
Inbound: Allow HTTP (port 80)
You must also allow:
Outbound: Allow ephemeral ports (1024–65535)
Otherwise:
Response will be blocked
Rule Structure
Each rule has:
- Rule number (priority)
- Protocol (TCP/UDP)
- Port range
- Allow / Deny
Example:
100 ALLOW TCP 80 0.0.0.0/0
110 DENY ALL
Lower number = higher priority
Default NACL vs Custom NACL
Default NACL:
- Allows all inbound
- Allows all outbound
Custom NACL:
- Denies everything by default
- You must define rules manually
NACL vs Security Groups
| Feature | NACL | Security Group |
|---|---|---|
| Level | Subnet | Instance |
| Type | Stateless | Stateful |
| Rules | Allow + Deny | Allow only |
| Complexity | Advanced | Simple |
Real-World Example
Public Subnet NACL:
- Allow HTTP/HTTPS from internet
- Allow response traffic
Private Subnet NACL:
- Allow traffic only from app layer
- Deny all external access
This creates:
- Layered security
- Controlled communication
When to Use NACL
Use NACL when:
- You want subnet-level protection
- You need to block specific IP ranges
- You want an extra security layer
Do NOT rely only on NACL—combine with Security Groups.
Best Practices
- Use NACL as an additional layer (not primary)
- Allow only required ports
- Keep rules simple
- Use proper rule numbering
- Always allow return traffic
Common Mistakes
- Forgetting outbound rules ❌
- Blocking return traffic ❌
- Overcomplicating rules ❌
- Relying only on NACL ❌
How NACL Fits in AWS Architecture
- VPC → network
- Subnets → segmentation
- Route Tables → traffic routing
- Security Groups → instance-level security
It acts as a second security barrier
Why NACL Matters
Without NACL:
- No subnet-level control
- Harder to block malicious traffic
With NACL:
- You get centralized subnet security
- Better control over traffic
Conclusion
A Network ACL is a powerful security feature in Amazon Web Services.
It:
- Controls subnet-level traffic
- Adds an extra layer of protection
- Works alongside Security Groups
Mastering NACL helps you design secure and production-ready cloud architectures.
As, India
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