Private vs Public IP ((Pv4) Fundamental Differences

Public IP:
Public IP means the machine can be identified on the internet (WWW)
Must be unique across the whole web (not two machines can have the same public IP).
Can be geo-located easily
Private IP:
Private IP means the machine can only be identified on a private network only
The IP must be unique across the private network
BUT two different private networks (two companies) can have the same IPs.
Machines connect to WWW using a NAT + internet gateway (a proxy)

Elastic IP

With 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.
You can only have 5 Elastic IP in your account (you can ask AWS to increase that).
Overall, try to avoid using Elastic IP:
They often reflect poor architectural decisions
Instead, use a random public IP and register a DNS name to it.
Or, as we'll see later, use a Load Balancer and don't use a public IP

Logical component in a VPC that represents a virtual network card
The ENI can have the following attributes:
- Primary private IPv4, one or more secondary IPv4
- One Elastic IP (IPv4) per private IPv4
- One Public IPv4
- One or more security groups
- A MAC address
You can create ENI independently and attach them on the fly (move them) on EC2 instances for failover
Bound to a specific availability zone (AZ)