At which layer of the OSI model does the Network Load Balancer function?

The Network Load Balancer (NLB) operates at Layer 4 of the OSI model, which is the Transport Layer. This layer is responsible for the transmission of data segments between systems. The NLB can handle millions of requests per second while maintaining ultra-low latencies, making it highly efficient for routing TCP and UDP traffic.

By functioning at Layer 4, the NLB makes routing decisions based on IP address and TCP/UDP port numbers. This means it does not inspect the contents of the messages but instead focuses on the information that can be gleaned from the packet headers. This is crucial for its ability to rapidly distribute incoming traffic across multiple targets, increasing the availability and reliability of applications.

In contrast, options associated with higher layers like Layer 7 deal with application-level data and include more complex processing, such as content-based routing and SSL termination. Layer 3, which focuses on network layer functionalities such as IP addressing, is also a lower level than where the NLB operates. Therefore, the choice of Layer 4 is justified for the functionalities offered by the Network Load Balancer.