When I started working with AWS, I believed that security groups were enough.
My EC2 instances were running, ports were controlled, and nothing seemed exposed.
So naturally, when I came across Network ACLs (NACLs), I assumed they were optional or redundant.
That assumption turned out to be incomplete.
This article explains what I misunderstood initially, how security groups and NACLs actually differ, and how my approach to AWS network security changed once things clicked.
Why I Thought Security Groups Were Sufficient
Security groups are introduced early when launching EC2 instances.
They are simple to configure and immediately useful.
At the time, my thinking was:
- Security groups control traffic
- I can restrict ports and IPs
- That should be enough
Because everything worked as expected, I didn’t question this setup further.
NACLs felt like something “advanced” that I could ignore for now.
What NACLs Actually Do
A Network ACL works at the subnet level, not the instance level.
Instead of protecting a single EC2 instance, a NACL controls:
- What traffic is allowed into a subnet
- What traffic is allowed out of a subnet
One key difference I didn’t understand early on:
- NACLs are stateless
- You must explicitly allow both inbound and outbound traffic
This was very different from security groups.
The Confusion I Had Initially
The main confusion came from overlap.
Both security groups and NACLs:
- Control traffic
- Use rules
- Sit inside a VPC
So my question was:
“Why would I need both?”
The answer is layered security — but that only makes sense once you understand where each one operates.
The Key Differences That Finally Made It Clear
Here’s what helped everything click for me:
Security Groups
Applied at EC2 instance level
Stateful (responses are automatically allowed)
Easier to manage
Ideal for application-level control
Network ACLs
Applied at subnet level
Stateless (inbound and outbound rules required)
Act as a first line of defence
Useful for broad network restrictions
Once I saw this as layers, not duplicates, it made sense.
When Security Groups Are Usually Enough
For small projects or learning environments:
- Single EC2 instance
- Limited exposure
- Controlled IP access
Security groups alone can be sufficient if configured properly.
This is why many beginners never touch NACLs — and that’s okay initially.
When NACLs Start to Matter
NACLs become important when:
- You have multiple instances in a subnet
- You want a baseline security rule for the network
- You need to block or allow traffic at a broader level
- You want defence in depth
Instead of trusting every instance configuration, the subnet itself enforces rules.
How I Approach Network Security Now
After understanding both, my approach changed:
- Use NACLs for high-level subnet controls
- Use security groups for fine-grained instance access
- Avoid relying on only one layer
- Review rules periodically, not just at setup
Security stopped being something I “set once” and started being something I designed intentionally.
Who This Is Useful For
This lesson is especially relevant if you are:
- Learning AWS networking
- Confused by overlapping security concepts
- Preparing for real-world cloud roles
- Moving from tutorials to actual architectures
Understanding this early prevents fragile designs later.
Final Thoughts
Security groups and NACLs are not competitors.
They solve different problems at different layers.
What I got wrong at first wasn’t a configuration mistake —
it was how I thought about security boundaries.
Once that changed, my AWS architectures became clearer, safer, and easier to reason about.
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