What is an Elastic Network Interface (ENI) in AWS?

Jay TilluJay Tillu
4 min read

Table of contents

In the world of cloud computing, having control over your networking setup is crucial for both performance and security. Amazon Web Services (AWS) provides a handy tool called the Elastic Network Interface (ENI) that lets you manage network settings with flexibility and ease. In this blog, we’ll explore what an ENI is, why it matters, and how you can use it in simple terms.

🚗 Think of ENI Like a Virtual Network Card

Just like your computer has a network card to connect to the internet, every virtual machine (EC2 instance) in AWS also needs a way to connect to a network. That’s where the Elastic Network Interface (ENI) comes in.

An ENI is a virtual network card for your EC2 instance. It helps your instance communicate with other resources over the network — whether that’s within AWS or the internet.

Key points:

  • Virtual: It’s not a physical device, but a software construct in AWS.

  • Attachable: You can attach or detach it from EC2 instances as needed.

  • Configurable: You can assign your own private IP addresses, public IP addresses, and security groups.

Core Components of an ENI

Every ENI comes with several configurable parts:

  1. Primary Private IPv4 Address

    • Automatically assigned when the ENI is created.

  2. Secondary Private IPv4 Addresses (optional)

    • You can add more private IPs for hosting multiple applications on one ENI.

  3. Elastic IP Address (optional)

    • A public, static IPv4 address that you can map to your ENI so it can be reached from the internet.

  4. Security Groups

    • Virtual firewalls that control inbound and outbound traffic to the ENI.

  5. MAC Address

    • A unique identifier assigned by AWS; it doesn’t change for the lifetime of the ENI.

  6. Description and Tags (optional)

    • Helpful for labeling ENIs with a name or purpose.

How ENIs Work in AWS

  • Attachment: You create an ENI and then attach it to an EC2 instance in the same Availability Zone.

  • Detachment: You can detach an ENI (provided the instance remains running) and attach it to another instance.

  • Multiple ENIs per Instance: Depending on instance type, you can have multiple ENIs. This helps segregate traffic (for example, web traffic on one ENI and database traffic on another).

🔧 Real-World Example

Let’s say you run a web app on an EC2 instance. You want to:

  • Allow users to access the app (public internet),

  • And also connect to a secure internal database (private network).

You can:

  • Use the primary ENI for public access,

  • Attach a second ENI connected to a private subnet for database access.

This setup keeps things secure, efficient, and organized.

Common Use Cases

  1. Network and Security Separation

    • Run public-facing services (e.g., a web server) on one ENI and private backend services on another.

  2. High Availability (HA)

    • Quickly move an ENI from a failed instance to a standby instance without reconfiguring IP addresses.

  3. Load Balancing without Elastic Load Balancer

    • Host multiple applications on different ENIs of the same instance to steer traffic separately.

  4. Monitoring and Management

    • Dedicate an ENI for monitoring tools or management traffic, isolating it from data-plane traffic.

🛑 Things to Remember

  • ENIs live within a single Availability Zone.

  • You can attach multiple ENIs to certain EC2 instance types.

  • Only one ENI can be the primary (used during instance launch).

  • Detaching and reattaching ENIs can help during instance replacement.

Best Practices

  • Tag Your ENIs: Use clear, consistent tags (e.g., Environment=Prod, Role=Web) to keep track in large environments.

  • Limit Permissions: Grant IAM users only the permissions they need to create, attach, or detach ENIs.

  • Use Security Groups Wisely: Apply the principle of least privilege—only open ports that are absolutely necessary.

  • Monitor ENI Usage: Use AWS CloudWatch to track network traffic per ENI and set alarms on unusual patterns.

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Written by

Jay Tillu
Jay Tillu

Hello! I'm Jay Tillu, an Information Security Engineer at Simple2Call. I have expertise in security frameworks and compliance, including NIST, ISO 27001, and ISO 27701. My specialities include Vulnerability Management, Threat Analysis, and Incident Response. I have also earned certifications in Google Cybersecurity and Microsoft Azure. I’m always eager to connect and discuss cybersecurity—let's get in touch!