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AWS Elastic Network Interfaces

Otherwise known as ENI are a virtual network card and are used to give EC2 instances network access and are able to be used outside of EC2 instances as well.

An AWS ENI is bound to single Availability Zone.

It is feasible for an EC2 to have one or more ENI associated with it

Each AWS ENI is associated with a primary private IPv4 IP addresses and one or more secondary IPv4 IP Addresses.
An AWS ENI is able to have a single Elastic IPv4 IP Address for each private IPv4 IP address as well as a single public IPv4 IP address

An AWS ENI can be associated with one or more Security Groups as well as a MAC Address.

It is possible to create an independent AWS ENI and attach the ENI on an ad-hoc basis to EC2 instances for failover