Tag: NAT Gateway

Introducing Azure Private Subnets: Enhancing Security by Disabling Default Outbound Access

Azure Networking Tips & Techniques - Part 1

Azure recently announced the general availability of Private Subnet, a new feature that allows you to disable the implicit outbound Internet connectivity for virtual machines in a subnet. In this blog post, we’ll cover:

  1. What Azure Private Subnets are and why they matter

  2. Key benefits of disabling default outbound access

  3. Step-by-step instructions to configure a private subnet via the Azure Portal

  4. Verifying that default outbound has been disabled

What Is an Azure Private Subnet?

Traditionally, when you create a subnet in an Azure Virtual Network (VNet) without any explicit outbound connectivity (such as a NAT gateway, Public IP, or Load Balancer), Azure automatically provides a default outbound access IP for those VMs. While this is convenient, it introduces an implicit egress path—VMs can communicate with public endpoints without you having explicitly configured any egress resources.

A Private Subnet in Azure is simply a subnet where this default outbound access is turned off. Consequently:

  • Any VM deployed within that subnet cannot reach the Internet by default.

  • You must explicitly configure an alternative egress mechanism if VMs need outbound connectivity (e.g., NAT Gateway, Standard Load Balancer, Firewall, or a Public IP assigned directly to the NIC).

By removing the implicit outbound IP, Azure Private Subnets enforce a “zero trust” approach: no VM can communicate externally until you grant it an explicit, auditable path.

Why Disable Default Outbound Access?

  1. Secure by Default
    Default outbound IPs are not customer-owned and can change unpredictably. By disabling implicit egress, you ensure VMs only send traffic externally when you explicitly allow it, reducing your attack surface.

  2. Prevent Data Exfiltration
    In regulated or highly sensitive environments (for example, PCI-DSS or HIPAA workloads), any unsolicited outbound route can pose compliance or security risks. Private Subnets eliminate unexpected data exfiltration channels.

  3. Encourage Explicit Egress Configuration
    When default outbound is disabled, you must provision a known, managed egress mechanism (like a NAT Gateway), which can be tightly monitored for logging, analytics, and cost control. This “explicit-over-implicit” model aligns with best practices for cloud network security.

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