Notes on Elastic Network Interfaces (ENIs) in AWS

Elastic Network Interfaces (ENIs) in AWS are a key component of Amazon EC2 instances, and they play a role in network connectivity and management.

1. Definition: Elastic Network Interfaces (ENIs) are virtual network interface cards that you can attach to Amazon EC2 instances.
2. Functionality: ENIs enable network connectivity for EC2 instances and are essential for various networking features.
3. Attachment: You can attach multiple ENIs to an EC2 instance, each serving a different purpose.
4. Primary ENI: Every EC2 instance is launched with a primary ENI that is created automatically.
5. Additional ENIs: You can attach additional ENIs to an EC2 instance, up to the instance type’s limits.
6. Use Cases: ENIs are used for various purposes, such as connecting to Virtual Private Clouds (VPCs), Elastic Load Balancers (ELBs), or Network Address Translation (NAT) gateways.
7. MAC Address: Each ENI has its own unique MAC address.
8. Private IP Addresses: ENIs can be assigned one or more private IP addresses from the associated subnet.
9. Elastic IP Addresses: You can associate Elastic IP addresses with a specific ENI to make them persistent.
10. Security Groups: ENIs can have their own security group rules to control inbound and outbound traffic.
11. Network ACLs: They can also be associated with Network Access Control Lists (NACLs) for additional network security.
12. Instance Termination: When an EC2 instance is terminated, all associated ENIs are automatically detached.
13. ENI Types: There are two types of ENIs: standard and elastic fabric adapter (EFA) ENIs.
14. Standard ENIs: Standard ENIs are used for general-purpose networking requirements.
15. Elastic Fabric Adapter (EFA) ENIs: EFA ENIs are specialized for high-performance computing (HPC) and tightly coupled, low-latency workloads.
16. Warm Pools: You can create warm pools of EC2 instances with pre-attached ENIs for faster scaling.
17. ENI Limitations: The number of ENIs you can attach to an EC2 instance depends on the instance type and the AWS region.
18. ENI Limit Increases: You can request a limit increase for ENIs if your use case requires more than the default limit.
19. ENI Detach/Attach: You can detach and attach ENIs to running instances without stopping or rebooting them.
20. Elastic Network Adapter (ENA): Many EC2 instances are launched with ENAs, which provide enhanced networking capabilities.
21. Bonds and Bonding Modes: You can bond multiple ENIs for redundancy and increased network throughput using bonding modes like active-passive or active-active.
22. Cross-VPC Attachments: You can attach an ENI from one VPC to an EC2 instance in another VPC within the same region using VPC Peering or Transit Gateway.
23. ENI Monitoring: You can monitor the network performance of ENIs using Amazon CloudWatch metrics.
24. ENI Lifecycle Hooks: You can use EC2 Auto Scaling lifecycle hooks to manage ENIs during scaling events.
25. EC2 Launch Templates: ENIs can be specified in EC2 launch templates for consistent ENI attachment during instance creation.
26. Resource Tagging: You can assign tags to ENIs for better resource management and tracking.
27. Security Group Rules: ENIs can be a part of multiple security groups, allowing you to define rules more granularly.
28. ENI Elastic Inference: ENIs can be used to attach Elastic Inference accelerators to EC2 instances for AI/ML workloads.
29. ENI Private DNS: ENIs support private DNS resolution, allowing instances to resolve private DNS names.
30. ENI Limits for Transit Gateway: When using Transit Gateway, there are specific limits on the number of ENIs that can be attached to instances in a VPC attachment.

Elastic Network Interfaces are a crucial part of configuring and managing the network connectivity of your EC2 instances in AWS, and understanding their capabilities is important for building scalable and secure architectures.