NLB IP target for ALB

While AWS does support using IP addresses as targets for Application Load Balancers (ALBs), there is no direct documentation stating that you can specifically add an internal Network Load Balancer (NLB) IP address as a target in an ALB.

The ability to use IP addresses as targets for ALBs allows for more flexibility in load balancing configurations, including the possibility of targeting resources that are not directly managed by AWS, such as on-premises servers. However, the specific use case of adding an NLB's IP address as a target for an ALB is not explicitly documented or confirmed as a supported configuration by AWS.

It's important to note that NLBs and ALBs have different use cases and operate at different network layers. NLBs operate at the transport layer (Layer 4) and are designed for handling TCP, UDP, and TLS traffic, while ALBs operate at the application layer (Layer 7) and are optimized for HTTP and HTTPS traffic.

If you need to combine the functionalities of both NLB and ALB, a more common and well-documented approach is to use an ALB as a target for an NLB. This configuration allows you to leverage the layer 7 routing capabilities of the ALB while still benefiting from the static IP addresses and other features of the NLB.

Given the lack of explicit documentation supporting the use of an NLB IP as a target for an ALB, it would be advisable to consult with AWS support or explore alternative architectures that are well-documented and supported for your specific use case.
Sources
Use Application Load Balancers as targets of a Network Load Balancer - Elastic Load Balancing
Elastic Load Balancing: Network Load Balancer now supports load balancing to IP addresses as targets for AWS and on-premises resources