A Network Acl Filters Traffic At The Ec2 Instance Level

How To Articles

As a technical expert, I am excited to share my knowledge about how network ACLs filter traffic at the EC2 instance level. Network Access Control Lists (ACLs) are an essential component of Amazon Web Services (AWS) that help control inbound and outbound traffic at the subnet level. They act as a firewall for EC2 instances, enabling fine-grained control over the network traffic flow.

When it comes to securing and managing network traffic within an EC2 instance, implementing network ACLs is a crucial step. These ACLs are stateless, meaning that they evaluate each packet independently based on the defined rules. Let’s dive into the details of how network ACLs work.

Understanding Network ACLs

Network ACLs are associated with subnets in AWS. Each subnet can have only one network ACL, but multiple subnets can share the same network ACL. These ACLs are evaluated in a sequential order to determine how traffic is allowed or denied.

Network ACLs consist of a set of numbered rules, with lower numbers being evaluated first. Each rule specifies the source and destination IP addresses, protocols, and port ranges. By default, all inbound and outbound traffic is denied unless explicitly allowed by the defined rules.

Inbound and outbound rules in network ACLs consist of numbered entries defining the allowed or denied traffic. These rules can be based on IP addresses, IP ranges, protocols, and port ranges. For example, you can create a rule to allow inbound SSH traffic (port 22) from a specific IP range, while denying all other traffic to the EC2 instance.

Working with Inbound and Outbound Rules

When an inbound packet reaches the network ACL, it is processed through each inbound rule in sequential order. The packet is compared to the rule criteria, and if a match is found, the action specified in the rule is performed. The actions can be either “allow” or “deny.” The first rule that matches the packet criteria is applied, and subsequent rules are skipped.

Similarly, when an outbound packet leaves the EC2 instance, it is evaluated against each outbound rule in sequential order. If a match is found, the action specified in the rule is applied. The outbound rules work in the same way as inbound rules, ensuring that only the desired traffic is allowed to leave the instance.

Benefits of Network ACLs

Network ACLs offer several benefits for securing EC2 instances and managing network traffic:

  1. Control over inbound and outbound traffic: Network ACLs allow you to define specific rules to allow or deny traffic as per your requirements. This provides granular control over the flow of network traffic at the instance level.
  2. Protection against unauthorized access: By configuring network ACLs to allow only necessary protocols and ports, you can protect your EC2 instance from unauthorized access and potential security threats.
  3. Flexibility in managing different subnets: Since network ACLs can be associated with multiple subnets, you can apply the same set of rules across different subnets, simplifying the management of network traffic within your AWS infrastructure.
  4. Performance optimization: Network ACLs are evaluated at the subnet level, making them more efficient for a large number of instances and traffic. They allow for faster processing of packets compared to security groups that operate at the instance level.


Network ACLs are a powerful tool for controlling inbound and outbound traffic at the EC2 instance level. By defining specific rules, you can ensure that only the desired traffic is allowed and unwanted traffic is denied. This enhances the security and manageability of your AWS infrastructure while providing flexibility and performance optimization.

Next time you set up an EC2 instance, make sure to configure network ACLs to safeguard your resources and maintain complete control over your network traffic.