2. Network Load Balancer

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Overview

Info

Network Load Balancer (NLB) operates at Layer 4 (Transport Layer) and routes traffic based on TCP, UDP, or TLS. It is designed for ultra-low latency, high throughput, and non-HTTP workloads.

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When to Use NLB?

Use an NLB when your application requires:

• Static IP addresses (Elastic IPs)
• Ultra-low latency
• Millions of concurrent TCP/UDP connections
• Non-HTTP protocols (TCP, UDP, TLS)
• Source IP preservation
• AWS PrivateLink integration

Tip

If you need static IPs for a web application:

• NLB provides Elastic IPs 3. IP Addresses in AWS

• You can place an ALB behind an NLB (or use AWS Global Accelerator) to combine:

  • Static IPs

  • Layer 7 routing

  • HTTP/HTTPS features

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ALB vs NLB

Feature	NLB	ALB
OSI Layer	Layer 4 (TCP, UDP, TLS)	Layer 7 (HTTP, HTTPS, WebSocket)
Routing	IP + Port	URL, Host, Headers, Cookies
Protocols	TCP, UDP, TLS	HTTP, HTTPS, WebSocket
Static IP	✅ Elastic IP	❌
Preserve Client IP	✅ Native	Via X-Forwarded-For
Performance	Ultra-high	High
Latency	Very Low	Higher (HTTP inspection)
TLS Termination	✅	✅
Advanced Routing	❌	✅
AWS WAF	❌	✅
PrivateLink	✅ Required	❌
Target Types	EC2, IP, ALB	EC2, IP, Lambda

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Why Do We Need NLB If We Already Have ALB?

Because ALB only get use to understands HTTP/HTTPS traffic.

NLB is required for workloads using protocols like:

• MySQL
• PostgreSQL
• Redis
• Kafka
• MQTT
• DNS
• SSH
• VPN
• Gaming servers

ALB cannot inspect or route these protocols.

Rule of thumb: HTTP is just one application-layer protocol 80 / 443. Many backend systems define their own optimized protocols over TCP, which is why a Layer 4 load balancer (NLB) is essential for those workloads.

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Layer Difference

flowchart LR
    Client --> NLB
    NLB -->|TCP / UDP / TLS| Backend

• Does not inspect application data.
• Simply forwards packets based on IP + Port.

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flowchart LR
    Client --> ALB
    ALB -->|"/users"| UserService
    ALB -->|"/orders"| OrderService
    ALB -->|"/payments"| PaymentService

• Reads the HTTP request.
• Supports path-based and host-based routing.

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Common Use Cases

Use NLB For	Reason
Gaming Servers	UDP + Low latency
Kafka / RabbitMQ	TCP traffic
Databases	Native TCP connections
Redis	TCP
VPN / SSH	Layer 4 protocols
AWS PrivateLink	Required
Firewall Whitelisting	Static IP support

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Architecture

flowchart LR
    Client -->|"TCP / UDP"| NLB
    NLB --> EC2_1
    NLB --> EC2_2
    NLB --> EC2_3

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DNS Lookup

nslookup <your-nlb-dns-name>.elb.<region>.amazonaws.com

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Key Interview Points

• Operates at Layer 4 (Transport Layer).
• Routes using IP Address + Port.
• Supports TCP, UDP, and TLS.
• Provides Elastic (Static) IPs.
• Preserves the original client IP.
• Designed for ultra-low latency and high throughput.
• Required for AWS PrivateLink.
• Does not support URL- or host-based routing (use ALB for that).

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Resources

• Deploying AWS Elastic Load Balancers | ALB and NLB

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