An instance brought up in a VPC with internet access is able to respond to traffic directed to an internet routable public IP assigned to it, as well as the internal subnet IP it has on it's only interface.

How is that accomplished?

edit for further information: This is an standard EC2 instanced created inside a VPC I have made that does not have a NAT instance.

The box has an public Elastic IP applied to it and can send and receive over this route, despite only having one interface with a private 10.x address.


An Elastic IP address is a property of network interfaces.

That is, it's bound by VPC to the elastic network interface attached to your instance. Your instance's IP stack is not aware of the Elastic IP address.

An Elastic IP address is accessed through the Internet gateway of a VPC.

The Internet Gateway VPC object is the logical entity that does the mapping -- the VPC network infrastructure does 1:1 static network address translation between the Elastic IP address and the associated private IP address that it's bound to. (By contrast, the NAT performed by NAT instances is 1:Many, dynamic port address translation.

Both citations are from docs.aws.amazon.com/AmazonVPC/latest/UserGuide/vpc-ip-addressing.html

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