How Can Azure Network Security Groups Be Used to Control Application Traffic?

Azure Network Security Groups (NSGs) provide a layer of network traffic filtering for Azure resources such as virtual machines and network interfaces. They allow organisations to define inbound and outbound security rules based on source, destination, protocol, port and direction. For example, a three-tier application may have a web tier, application tier and database tier. Instead of allowing unrestricted traffic between them, NSGs can enforce rules such as allowing HTTPS traffic to the web tier, permitting only the required application ports between the web and application tiers, and restricting database access to the application subnet. A good design principle is least privilege—allow only the traffic that the application actually requires. NSGs are stateful, so return traffic for an allowed connection is generally permitted automatically. However, NSGs should not be treated as the complete security architecture. They work alongside Azure Firewall, Application Gateway/WAF, private endpoints, routing controls and identity-based security depending on the scenario. For troubleshooting, engineers should examine rule priorities, effective security rules, subnet versus NIC-level associations, service tags and actual application ports. A common mistake is opening broad ranges such as 0.0.0.0/0 when a narrower source range would be sufficient. The key takeaway: NSGs control network-level traffic flow; they do not replace application authentication, authorisation or other security controls.