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An elastic network interface is defined as a virtual network interface https://docs.aws.amazon.com/vpc/latest/userguide/VPC_ElasticNetworkInterfaces.html.
I was wondering what use cases exist of this. Why would not I simply put directly an instance instead of having to use the eni, and attach it to an instance.
Thks!
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AWS Network Interface or Elastic Network Interfaces (ENIs) are virtual network cards attached to EC2 instances that help facilitate network connectivity for instances. Having two or more of them connected to an instance permits it to communicate on two separate subnets.
Essentially, ENIs are virtual network cards you can attach to your EC2 instances. They are used to enable network connectivity for your instances, and having more than one of them connected to your instance allows it to communicate on two different subnets.
By using an Elastic IP address, you can mask the failure of an instance or software by rapidly remapping the address to another instance in your account. Alternatively, you can specify the Elastic IP address in a DNS record for your domain, so that your domain points to your instance.
Elastic Network Interface (ENI)
An elastic network interface (ENI) is a virtual network interface that you can attach to an instance in a VPC. An ENI can include one public IP address, which can be auto-assigned to the elastic network interface for eth0 when you launch an instance, but only when you_____.
Elastic Network Interfaces. An elastic network interface (referred to as a network interface in this documentation) is a logical networking component in a VPC that represents a virtual network card.
It also makes it possible for you to connect to the operating system running on your instance to manage it. As the name suggests, an ENI performs the same basic function as a network interface on a physical server, although ENIs have more restrictions on how you can configure them.
An elastic network interface is a logical networking component in a VPC that represents a virtual network card. It can include the following attributes: One or more secondary private IPv4 addresses from the IPv4 address range of your VPC You can create and configure network interfaces and attach them to instances in the same Availability Zone.
Decoupling the ENI from the EC2 instance allows you far more flexibility.
You can connect several ENIs to the same EC2 instance and be attached to different subnets, for example:
ENI A connected to a public-facing subnet
ENI B connected to an internal private subnet
You could, for example, attach an ENI to a running EC2 instance, or you could have it live after the EC2 instance is deleted.
As a form of "cheap" high availability: Attach a ENI to an EC2 instance; if the instance dies launch another one and attach the ENI to it. Traffic flow will only be interrupted for a short period.
There are many scenarios where this flexibility comes in handy.
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