When you run
nslookup dualstack.app.elb.us-east-2.amazonaws.com you are querying DNS and it's returning an IP address for an elastic load balancer node in each of the registered Availability Zones. Your client app then decides which IP to use for its request.
If you are requesting against the elastic load balancer node IP directly and not receiving a response, that means there are no targets responding to the node's request in that availability zone
Quoted for reference with formatting fixed
With Application Load Balancers, the load balancer node that receives the request uses the following process:
- Evaluates the listener rules in priority order to determine which
rule to apply.
- Selects a target from the target group for the rule action, using
the routing algorithm configured for the target group. The default
routing algorithm is round robin. Routing is performed independently
for each target group, even when a target is registered with
multiple target groups.
With Network Load Balancers, the load balancer node that receives the connection uses the following process:
- Selects a target from the target group for the default rule using a flow hash algorithm. It bases the algorithm on:
- The protocol
- The source IP address and source port
- The destination IP address and destination port
- The TCP sequence number
- Routes each individual TCP connection to a single target for the life of the connection. The TCP connections from a client have different source ports and sequence numbers, and can be routed to different targets.
With Classic Load Balancers, the load balancer node that receives the request selects a registered instance as follows: