Introduction to Azure Virtual Networks (VNets)

Azure Virtual Network (VNet) is the fundamental building block for your private network in Azure. VNet enables many types of Azure resources, such as Azure Virtual Machines (VMs), to securely communicate with each other, with the internet, and with your on-premises networks. VNet is similar to a traditional network that you would operate in your own datacenter, but with additional benefits of the Azure infrastructure such as scalability, availability, and isolation.

This deep dive will explore the core concepts, functionalities, and best practices for designing and implementing secure and scalable virtual networks in Azure.

VNet Core Concepts

At its heart, an Azure VNet is a logical isolation of the Azure cloud dedicated to your subscription. Key concepts include:

  • Regions: VNets are region-specific. Resources deployed in a VNet must be in the same region as the VNet.
  • Global Reach: While VNets are regional, you can connect VNets across different regions using VNet peering or VPN gateways.
  • IP Addressing: VNets use private IP address spaces that you define.
  • Isolation: Resources within a VNet are isolated from resources in other VNets by default.

Address Spaces and Subnets

When you create a VNet, you define one or more IP address spaces using Classless Inter-Domain Routing (CIDR) notation. These address spaces can be public or private. However, it's a best practice to use private IP address spaces (RFC 1918) for your VNets to avoid conflicts with public IP addresses.

A VNet address space can be divided into one or more subnets. Subnets allow you to segment your VNet into smaller, manageable network segments. Each subnet must have an address range that is a subset of the VNet's address space. Azure reserves the first four and the last IP address in each subnet for IP-level functions.


{
  "name": "myVNet",
  "location": "eastus",
  "properties": {
    "addressSpace": {
      "addressPrefixes": [
        "10.0.0.0/16"
      ]
    },
    "subnets": [
      {
        "name": "subnet-frontend",
        "properties": {
          "addressPrefix": "10.0.1.0/24"
        }
      },
      {
        "name": "subnet-backend",
        "properties": {
          "addressPrefix": "10.0.2.0/24"
        }
      }
    ]
  }
}

Network Security Groups (NSGs)

Network Security Groups (NSGs) are fundamental to securing your Azure resources. They act as a distributed firewall at the network interface (NIC) or subnet level, allowing you to define rules that permit or deny network traffic. NSGs contain a list of security rules that allow or deny inbound and outbound traffic based on:

  • Source and destination IP address/CIDR range
  • Source and destination port
  • Protocol (TCP, UDP, ICMP, Any)
  • Action (Allow, Deny)

Azure provides some default NSG rules, but you can create custom rules to enforce your specific security policies.

Routing and Route Tables

Azure automatically routes traffic between subnets within a VNet. For more control over traffic flow, you can create custom route tables. Route tables allow you to define paths for network traffic to override Azure's default system routes. This is particularly useful for scenarios like:

  • Forcing traffic through a Network Virtual Appliance (NVA) for inspection or firewalling.
  • Directing specific traffic to an on-premises network via a VPN gateway.

Routes can be defined with a destination prefix and a next hop type (e.g., Virtual Appliance, Virtual Network Gateway, Internet).

Connectivity Options

Azure VNets offer various options for connecting resources and extending your network:

  • VNet Peering: Connects two Azure VNets together. Traffic between them is routed directly through the Azure backbone network, eliminating the need for a VPN gateway. This is a highly performant and cost-effective solution for inter-VNet communication within the same or different regions.
  • VPN Gateway: Connects your on-premises network to your Azure VNet over an encrypted tunnel (IPsec/IKE). This allows hybrid cloud scenarios, enabling seamless access to on-premises resources from Azure and vice versa.
  • ExpressRoute: A dedicated, private connection between your on-premises infrastructure and Azure. It offers higher bandwidth, lower latency, and greater reliability than VPN connections.

Advanced Topics in Azure Networking

Beyond the basics, Azure VNets support advanced features for complex networking requirements:

Private Endpoints: Allow you to access Azure PaaS services (like Azure SQL Database, Storage Accounts) from your VNet using private IP addresses, enhancing security by keeping traffic within the Azure network.
Service Endpoints: Extend your VNet's private address space and identity to Azure services, enabling secure access to services like Azure Storage and Azure SQL Database over the Azure backbone network.
  • Load Balancing: Azure Load Balancer and Application Gateway provide high availability and scalability for your applications.
  • Firewall: Azure Firewall is a managed, cloud-based network security service that protects your VNet resources.
  • DNS: Azure provides Azure Private DNS Zones for custom domain names within your VNet.

Conclusion

Azure Virtual Networks provide a robust and flexible platform for building secure, scalable, and highly available network infrastructures in the cloud. Understanding core concepts like address spaces, subnets, NSGs, routing, and connectivity options is crucial for designing effective Azure solutions. By leveraging advanced features, you can further enhance security, performance, and integration with your on-premises environments.

Continue exploring the official Azure documentation for more in-depth guides and specific configuration details.