An Elastic Network Interwork, or ENI, is the virtual network card that you attach your EC2 instances to. These are not bound to your instances and they can be freely moved. Let’s say for example you have an ENI on one instance and you want to move it to another, that is something that is very much possible. You have one primary IP on the ENI but you can set multiple secondary IPs as well for failover capabilities. This post was a rather short one I just wanted to provide a brief overview of ENI. Final note, if you create an ENI it is bound to the AZ it is created in.