Designing a secure network in the cloud is one of the most important responsibilities for security engineers, cloud architects, and administrators. As organizations adopt cloud platforms, they need to ensure their workloads remain protected from unauthorized access, malicious actors, and misconfigurations. This is where Azure Firewall and Network Security Groups (NSGs) play a central role. Both are essential tools for segmentation, traffic filtering, and enforcing a strong security posture across cloud environments.

In this blog, we will explore how to design secure network architectures using Azure Firewall and NSGs, how they complement each other, and the best practices you should be ready to discuss during interviews. We’ll proceed step-by-step, starting with foundational concepts and gradually moving into architectural design patterns.

Understanding the Foundations of Secure Network Design

Before diving into Azure Firewall and NSGs, it is helpful to understand the core principles behind a secure cloud network. Concepts such as segmentation, least privilege, and traffic control form the backbone of any architecture.

To build on these fundamentals, we now move into the key components used in Azure to enforce these principles.

Overview of Network Security Groups (NSGs)

NSGs are one of the most commonly used security controls in Azure. They act like virtual firewalls that filter network traffic at the network interface (NIC) or subnet level.

How NSGs Work

NSGs allow you to define rules for both inbound and outbound traffic.

Each rule includes:

  • Source and destination
  • Port and protocol
  • Priority
  • Action (allow/deny)

These rules help restrict access between workloads and ensure that communication happens only where needed.

Why NSGs Are Important

  • Lightweight and fast
  • Easy to implement
  • Ideal for east-west traffic filtering
  • Perfect for enforcing micro-segmentation
  • Essential for internal workload isolation

Now that we understand NSGs, let’s introduce a stronger, more centralized control mechanism—Azure Firewall.

Introduction to Azure Firewall

Azure Firewall is a fully managed, stateful network security service. Unlike NSGs, which provide basic network-level filtering, Azure Firewall provides deep packet inspection, rule collections, threat intelligence filtering, and application-level control.

Key Features of Azure Firewall

  • Stateful traffic inspection
  • Application and network rules
  • Threat Intelligence-based filtering
  • DNAT and SNAT capabilities
  • Full logging through Azure Monitor
  • Support for FQDN filtering and IDPS (depending on SKU)

With this foundation, it’s easier to see how Azure Firewall and NSGs complement each other in secure network design.

Azure Firewall vs NSGs: What’s the Difference?

To design an interview-ready secure architecture, you must understand the role of each component.

NSGs Handle Local Traffic Filtering

  • Enforce rules at NIC/subnet level
  • Ideal for workload-level restrictions
  • Manage VM-to-VM communication
  • Prevent unnecessary lateral movement

Azure Firewall Handles Centralized Traffic Control

  • Controls north-south and east-west traffic
  • Provides advanced threat protection
  • Central inspection point
  • Ideal for hub-and-spoke architectures

With these differences in mind, let’s shift toward the architectural patterns that combine both tools.

Designing Secure Network Architectures in Azure

Transitioning from individual components to an actual architecture helps align theory with practical implementation. Let’s explore how Azure Firewall and NSGs work together in an enterprise setup.

Hub-and-Spoke Architecture with Azure Firewall

The hub-and-spoke model is one of the most popular architectures for secure network design.

How It Works

  • The hub contains shared services, including Azure Firewall, VPN gateways, and monitoring tools.
  • The spokes contain workloads such as virtual machines, app services, and databases.
  • All traffic from the spokes is routed to the hub for inspection.

Why It’s Secure

  • Centralized management
  • Forced tunneling through Azure Firewall
  • Cleaner segmentation
  • Streamlined security monitoring

Now that we’ve seen the overall structure, let’s dive deeper into segmentation.

Network Segmentation Using NSGs

Segmentation is essential for reducing the attack surface. Even inside a spoke, you should break resources into smaller trust zones.

Example Segments

  • Application subnet
  • Database subnet
  • Management subnet

Benefits of Segmentation

  • Strict control of east-west traffic
  • Minimizes risk from compromised workloads
  • Simplifies incident response

With segmentation in place, Azure Firewall can handle higher-level protection.

Combining Azure Firewall and NSGs in a Secure Architecture

A layered security model often called defense in depth, is most effective in modern cloud networks.

How They Work Together

  • NSGs block unnecessary local traffic.
  • Azure Firewall inspects traffic centrally for advanced threats.
  • Route tables direct traffic from subnets to the firewall.
  • Segmentation prevents lateral movement.

Example Layered Workflow

  • VM communication → filtered by NSG
  • Subnet traffic → routed to Azure Firewall
  • External outbound traffic → inspected via Firewall
  • Logs → forwarded to SIEM for analysis

This combined approach produces a robust and scalable network design.

Traffic Filtering Strategies with Azure Firewall

Traffic filtering is a critical interview topic. Azure Firewall supports two major rule types.

Network Rules

Ideal for IP-based filtering such as:

  • Allowing specific ports
  • Controlling outbound access
  • Restricting internal communication

Application Rules

Used for:

  • URL and FQDN filtering
  • Web access restrictions
  • SaaS traffic control

Let’s continue by discussing additional considerations for securing cloud networks.

Implementing Least Privilege Access

Least privilege ensures that only the required traffic is allowed. This improves security by minimizing the potential for misuse.

How to Apply Least Privilege

  • Deny all inbound by default
  • Open only required outbound ports
  • Use NSGs for workload-level enforcement
  • Use Azure Firewall for application filtering

Now that we’ve covered least privilege, it’s important to explore logging and monitoring.

Monitoring and Logging for Secure Cloud Networks

Security is incomplete without continuous visibility.

Azure Firewall Logging

Azure Firewall logs:

  • Allowed and denied traffic
  • Application-level events
  • Threat detection alerts

NSG Flow Logs

NSG flow logs track:

  • Traffic patterns
  • Anomalies
  • Lateral movement attempts

Send both to:

  • Microsoft Sentinel
  • Log Analytics Workspace
  • Any SIEM tool

This gives SOC teams full visibility into cloud traffic.

Best Practices for Secure Network Design in Azure

Before we conclude, let’s summarize the top recommended practices.

Key Best Practices

  • Use Azure Firewall for centralized traffic filtering
  • Apply NSGs at NIC and subnet levels
  • Build a hub-and-spoke architecture
  • Enforce segmentation across environments
  • Monitor traffic using Azure Firewall logs and NSG flow logs
  • Restrict outbound traffic with FQDN filtering
  • Use threat intelligence in Azure Firewall
  • Apply least privilege across all rules
  • Review and refine rules regularly

Conclusion

Designing secure network architectures with Azure Firewall and Network Security Groups requires a clear understanding of how each component operates and how they complement each other. NSGs provide essential subnet and NIC-level filtering, while Azure Firewall offers centralized, stateful, and intelligent inspection. When combined with segmentation, logging, and least privilege, these tools help build a strong, scalable, and resilient cloud security architecture.

This knowledge is not only crucial for real-world implementation but also incredibly valuable during interviews where cloud security and network design skills are often tested. Mastering these concepts will help you articulate clear architectural explanations and demonstrate practical expertise.