Network Load Balancer (NLB)
- What it is
- Layer 4 (TCP, UDP, TLS) load balancer. Routes on connection information only — it never inspects the payload.
- Built for extreme performance: millions of requests per second at ultra-low latency.
- The differentiators
- Static IP per Availability Zone, and supports Elastic IPs. The reason to pick it when clients must allowlist fixed IPs.
- Preserves the client source IP by default — the backend sees the real caller with no
X-Forwarded-Forparsing. - Handles UDP and non-HTTP protocols: gaming, VoIP, IoT/MQTT, syslog, database connections.
- Scales instantly with no pre-warming.
- What it cannot do
- No path or host-based routing, no header inspection, no AWS WAF integration, no Cognito authentication. All of that requires Application Load Balancer (ALB).
- Common pattern
- NLB in front of ALB when you need both static IPs and Layer 7 routing.
- Exam signal
- “Static IP”, “millions of requests”, “extreme low latency”, “UDP/TCP non-HTTP” → NLB.