How Amazon ELB identifies new instances added

Question

I am working on using a elastic load balancer along with AWS Auto scaling. I do have a setup in which instances will be scaled up/down automatically based on NetworkIn and it is working fine. I have a couple of questions regarding ELB.

1. How a fresh auto-scaling launched instance is registered with the ELB automatically? I know we will give the load balancer name while creating the auto-scaling group; I need to know the real 'how'.
2. Can we have multiple private IPs of instances run different applications and all of them visible to ELB?

Explanation for the 2). Lets say I configure the instances so that they will have multiple private IPs at the time of launch. Could they be exposed to ELB rather than the Public IP of the machine? Can ELB read the private IPs of the instances that are launched under it?

Answer 1

How a fresh auto-scaling launched instance is registered with the ELB automatically? I know we will give the load balancer name while creating the auto-scaling group; I need to know the real 'how'.

My guess is it makes an API call RegisterInstancesWithLoadBalancer. You can do that too in your own code. It does not have to be through Auto Scaling

Can we have multiple private IPs of instances run different applications and all of them visible to ELB?

Well, ELB does not care about the ip address at all. It goes by the Instance Id. Unless it is on a VPC and uses ENI. However, ELB routes traffic only to ip address attached to eth0

Update:

Note:

When you register a multi-homed instance (an instance that has an elastic network interface (ENI) attached) with your load balancer, the load balancer will route traffic to the primary IP address of the instance (eth0).

Source: ELB Developer Guide