Exactly right — you’ve captured most of the key distinctions perfectly 👏
Let’s go over Amazon EC2 Security Groups and Network ACLs (NACLs) in a structured way so you can remember how and when to use each.
🧩 1. What is a Security Group (SG)?
A Security Group acts as a virtual firewall for EC2 instances.
It controls inbound and outbound traffic to and from individual instances.
Key Points
Operates at the instance level (ENI — Elastic Network Interface).
Stateful:
If you allow inbound traffic, the return outbound traffic is automatically allowed (and vice versa).Supports only “Allow” rules.
Rules evaluated collectively:
If any rule allows the traffic, it’s permitted.You must explicitly attach SGs to instances.
Example Use Case:
Allow HTTP (80) and SSH (22) traffic to a web server instance.
🧱 2. What is a Network ACL (NACL)?
A Network Access Control List acts as a firewall at the subnet level.
It controls traffic entering or leaving a subnet.
Key Points
Operates at the subnet level.
Stateless:
You must explicitly allow return traffic for each request.Supports both “Allow” and “Deny” rules.
Rules processed in ascending numerical order (rule numbers).
Automatically applied to all resources in that subnet.
Example Use Case:
Block a specific IP range (e.g. malicious IPs) for an entire subnet.
⚖️ 3. Security Group vs NACL — Comparison Table
| Feature | Security Group (SG) | Network ACL (NACL) |
|---|---|---|
| Level of Operation | Instance / ENI level | Subnet level |
| Statefulness | ✅ Stateful | ❌ Stateless |
| Rule Type | Only Allow rules | Both Allow and Deny rules |
| Default Behavior | Deny all inbound, allow all outbound | Allow all inbound/outbound by default (modifiable) |
| Rule Evaluation | All rules evaluated, one allow = allow | Rules processed in order (lowest to highest number) |
| Return Traffic | Automatically allowed | Must be explicitly allowed |
| Attachment | Applied to instances explicitly | Automatically applies to all instances in subnet |
| Best For | Fine-grained instance-level control | Broad subnet-level control or IP blocking |
🌐 4. Example Architecture
┌─────────────────────────────┐
│ Internet │
└────────────┬────────────────┘
│
┌────────▼────────┐
│ Internet Gateway│
└────────┬────────┘
│
┌────────────▼────────────┐
│ Public Subnet │
│ (Has NACL rules) │
│ │
│ ┌────────────────┐ │
│ │ EC2 Instance │ │
│ │ (Has SG rules) │ │
│ └────────────────┘ │
└─────────────────────────┘
Flow:
Traffic first hits the NACL at the subnet boundary.
If NACL allows it → traffic moves to the instance.
Then Security Group decides if the instance can accept it.
🧠 5. Remember It Like This
| Analogy | Description |
|---|---|
| NACL → Neighborhood Gate | Controls who can even enter the area (subnet). |
| Security Group → House Door | Controls who can come into your specific house (instance). |
✅ 6. Practical Design Tip
In most AWS setups:
Use Security Groups for regular instance-level access control (e.g., web, SSH, DB ports).
Use NACLs as an additional layer of security for broader rules, like blocking IP ranges or limiting entire subnet traffic.
Would you like me to generate a visual diagram (image) that shows how traffic flows through NACL → Security Group → Instance with arrows and rule examples (e.g., inbound 80 allowed, outbound denied)? It makes this concept super easy to recall during exams or interviews.
