How does nlb work

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Last updated: April 8, 2026

Quick Answer: Network Load Balancer (NLB) is an AWS service that operates at the transport layer (Layer 4) of the OSI model, distributing incoming TCP/UDP traffic across multiple targets like EC2 instances. It can handle millions of requests per second with ultra-low latency, automatically scaling its capacity based on traffic. NLB uses a single static IP address per Availability Zone and supports both internet-facing and internal load balancing, making it ideal for high-performance applications.

Key Facts

AWS Network Load Balancer launched in 2017
Operates at Layer 4 (transport layer) of OSI model
Can handle millions of requests per second
Provides static IP addresses for each Availability Zone
Supports both TCP and UDP protocols

Overview

AWS Network Load Balancer (NLB) is a managed load balancing service introduced by Amazon Web Services in 2017 as part of their Elastic Load Balancing portfolio. Unlike its predecessor, the Classic Load Balancer, NLB operates exclusively at the transport layer (Layer 4) of the OSI model, making it specifically designed for TCP and UDP traffic. This architectural choice allows NLB to handle extreme performance requirements, including millions of requests per second while maintaining ultra-low latencies. The service was developed in response to growing demand for high-performance, low-latency load balancing solutions, particularly for applications requiring persistent connections, such as gaming servers, financial trading platforms, and real-time streaming services. NLB's design reflects AWS's focus on providing specialized tools for different workload types within their cloud ecosystem.

How It Works

Network Load Balancer functions by distributing incoming TCP/UDP traffic across multiple targets, such as Amazon EC2 instances, containers, or IP addresses. When a client initiates a connection to the NLB's IP address, the load balancer selects a healthy target from the registered targets using a flow hash algorithm based on protocol, source IP address, source port, destination IP address, destination port, and TCP sequence number. This ensures that all packets from a particular connection are routed to the same target, maintaining connection persistence. NLB operates at the connection level rather than the request level, meaning it doesn't inspect application-layer content. Each NLB provides a single static IP address per Availability Zone, which remains constant even as the load balancer scales. Health checks monitor target availability, automatically rerouting traffic away from unhealthy targets. The service supports both internet-facing and internal load balancing configurations.

Why It Matters

Network Load Balancer is crucial for applications requiring extreme performance, low latency, and static IP addresses. Its ability to handle millions of requests per second makes it essential for high-traffic websites, gaming platforms, and financial applications where every millisecond counts. The static IP addresses per Availability Zone enable predictable network configurations and simplify firewall rules. NLB's support for long-lived TCP connections benefits applications like database clusters, IoT device communications, and real-time analytics systems. By offloading the complexity of traffic distribution and health monitoring to AWS's managed service, organizations can focus on application development rather than infrastructure management. This service represents a key component in building scalable, resilient architectures on AWS, particularly for workloads that cannot tolerate the additional latency of application-layer processing.