Elastic Network Interfaces (ENI) – Overview and Hands-On

An Elastic Network Interface (ENI) is a virtual network card that can be attached to an EC2 instance.
It allows communication between instances, VPCs, and external networks within AWS.
Each ENI represents a virtual network adapter that can have multiple IP addresses, MAC address, and security groups.

Key Components of an ENI

Component	Description
Primary Private IP	Automatically assigned when the ENI is created; cannot be removed.
Secondary Private IPs	Additional IPs that can be manually added to the same ENI.
Elastic IP	Can be associated with any of the private IPs for internet access.
MAC Address	Unique hardware identifier assigned automatically.
Security Groups	One or more security groups attached to control traffic.
Description	Optional text to identify the purpose of the ENI.

High Availability
- Create multiple ENIs in different subnets.
- Attach them to standby instances for quick failover in case of failure.
Network Management
- Use multiple ENIs to separate management, data, and monitoring traffic.
Security Isolation
- Assign different security groups to different ENIs for better control.
Load Balancing or Multi-IP Applications
- Use secondary IPs for multiple applications running on a single instance.
Move Network Interface Between Instances
- Detach an ENI from one instance and attach it to another (useful for quick recovery).

Type	Description
Primary ENI (eth0)	Automatically created when an instance is launched.
Secondary ENI (eth1, eth2, etc.)	Can be manually attached to an instance. Useful for custom routing or isolation.