Figure one shows the route table my route table:

An Internet Gateway (IGW) is built to automatically: scale horizontally, be redundant, and have a and high degree of availability.

An Amazon IGW is a VPC component that allows communication between instances in your Amazon VPC and the Internet by providing a target in your Amazon VPC route tables for Internet-routable traffic.

An IGW performs network address translation for instances that have been assigned public IP addresses as Amazon EC2 instances within an Amazon VPC are not aware of their public IP addresses.

For traffic that is sent from the instance to the Internet, the IGW translates the reply address to the instance's public IP address / Elastic IP and maintains the mapping of the instance’s private IP and public IP addresses.

When an instance receives traffic from the Internet, the IGW translates the destination address (public IP address) to the instance's private IP address and forwards the traffic to the Amazon VPC.

To create a public subnet with Internet access:

Attach an IGW to your Amazon VPC. Create a subnet route table rule to send all non-local traffic ( 0) to the IGW.
Setup your security group and network ACLs rules to allow relevant traffic to flow to and from your instance.

Assign a public IP address or EIP address to enable an Amazon EC2 instance to send and receive traffic from the Internet. A public IP is created by default when you create an EC2 instance but is ephemeral if you don’t use an EIP.

You can route to all destinations not explicitly known to the route table ( 0), or you can route to a narrower range of IP addresses IP addresses like: of your company's LAN or WAN.

To summarize:

You can only have ONE IGW per VPC.  Your IGW is assigned and referenced in your VPCs routing table.

One or more subnets (Availability Zones) those subnets can then reference this route table and are then allowed internet access.

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