Associate \28 Public subnet with an EC2 interface

Hi there,

The first four IP addresses and the last IP address in each AWS subnet CIDR block are not available for your use, and they cannot be assigned to a resource, such as an EC2 instance. This means you will only have 13 usable IPs in a /28

To answer the second part of your question, here are the steps to assign multiple IP addresses from your BYOIP subnet to a single EC2 instance interface:

1. Create a new EC2 instance and attach an Elastic Network Interface (ENI) to it. Use a subnet in your VPC that has a route to the internet gateway.
2. Modify the ENI to assign a primary private IP address in the VPC subnet range. This will be the primary private IP for the instance.
3. Under the network interface properties, change the setting for "Secondary private IPv4 addresses" to assign up to 15 additional secondary IPs. These must be within the CIDR range of your VPC subnet.
4. For each secondary private IP, associate it with an IP address from your BYOIP public subnet using the CLI:

aws ec2 assign-private-ip-addresses --network-interface-id ENI_ID --secondary-private-ip-addresses IP_1,IP_2 etc

5. Allocate an Elastic IP for each public IP you associated and associate it with the corresponding secondary private IP on the ENI.

This will result in the ENI having a primary private IP, up to 15 secondary private IPs, and a corresponding Elastic IP for each secondary IP. Your instance will then have 16 public IP addresses assigned from your BYOIP range, if your subnet can accomodate this.