ALB vs NLB

Overview

What is being compared

Both are AWS load balancers, and each solves a distinct set of problems. The Application Load Balancer works at Layer 7: it terminates HTTP and HTTPS connections and routes on content. The Network Load Balancer works at Layer 4: it passes TCP and UDP connections through to targets with very low latency and without modifying packet headers.

• OSI layer — 7, HTTP and HTTPS — 4, TCP and UDP
• Connection — Terminated and inspected — Passed through without inspection
• Routing — Path, host, query string, headers — Flows to targets
• Features — Sticky sessions, authentication, WebSocket, AWS WAF — TLS pass-through, non-HTTP protocols
• Client IP at target — In X-Forwarded-For — Preserved natively
• IP addresses — Dynamic, reach it by DNS name — Static Elastic IP per AZ
• Latency — Adds 1 to 5 ms for HTTP parsing and rules — Sub-millisecond

Layer 7 versus Layer 4 routing

The ALB ends the client's connection and opens its own to the target, so it can read the request and must carry the client IP in a header. The NLB hands the same flow to the target, so the target sees the client directly.

> [!NOTE] AWS WAF attaches only to ALB and CloudFront, never to NLB.

> [!TIP] Both load balancers side by side, a partner call through an NLB in front of an ALB, and the choice between them play step by step in Visual Explanation below.

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