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This question is more theoretical. I am studying to certify in AWS, I am reading for the second time the certified solutions architect study guide of aws and it is not clear to me what is the use that can be given to an ENI, I understand that they are used to connect a VPC with other services. Could someone give me an explanation "for dummies" and some examples of use? Thank you.

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ENI is a network interface, which embodies some configuration, which can include IP addresses, an elastic IP (EIP), various security groups (firewall rules), a MAC address, a source/destination check flag, and a description. An EIN can be attached to an EC2 instance (a virtual server); the server that it is attached to can change over time.

This allows you to have a long-lived ENI that over its lifetime may be attached to many short-lived servers. We have ENIs that have not changed for years that are attached to EC2 instances that are replaced every couple of weeks.

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  • but then you have 2 private ip attached to the the EC2 , how will you make sure to use the IP of ENI (eth1) and not eth0
    – ETL_Devs
    Apr 25, 2020 at 14:47
  • In this scenario, the EIP (that is associated with the ENI) is often used as the public IP for the "service" (which is provided by the EC2 instance that the ENI is currently associated to). There is usually no need to ensure that traffic goes through any particular network interface.
    – Rob
    Apr 25, 2020 at 17:39

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