Understanding Azure Virtual Network Address Space

Azure Virtual Network (VNet) address space is a fundamental concept for designing your cloud network infrastructure. It defines the range of IP addresses that can be used within your virtual network and its connected resources. Properly planning your VNet address space is crucial for avoiding IP address conflicts, enabling seamless connectivity between resources, and facilitating future network expansion.

What is VNet Address Space?

The VNet address space is a collection of one or more non-overlapping private IP address prefixes that are assigned to a virtual network. These prefixes are defined using Classless Inter-Domain Routing (CIDR) notation. For example, 10.0.0.0/16 represents a range of IP addresses from 10.0.0.0 to 10.0.255.255.

Key Considerations for Address Space Planning

• Private IP Address Ranges: Azure supports RFC 1918 private IP address ranges. The most commonly used ranges are:
  • 10.0.0.0/8 (10.0.0.0 to 10.255.255.255)
  • 172.16.0.0/12 (172.16.0.0 to 172.31.255.255)
  • 192.168.0.0/16 (192.168.0.0 to 192.168.255.255)
• Non-Overlapping Prefixes: It is critical that the address spaces of connected VNets (e.g., through VNet peering or VPN Gateway) do not overlap. Overlapping address spaces will prevent resources from communicating effectively.
• Future Growth: Plan for future growth. Consider the number of subnets you might need, the types of resources you'll deploy, and potential future VNet peering requirements. A larger initial address space can prevent costly re-architecting later.
• On-Premises Connectivity: If you plan to connect your VNet to your on-premises network via a VPN or Azure ExpressRoute, ensure that your VNet address space does not overlap with your on-premises IP address ranges.
• Service Limit: The maximum number of address prefixes you can associate with a VNet is 50.

Adding and Managing Address Space

When you create a virtual network, you define its initial address space. You can add more address space prefixes or remove existing ones after the VNet has been created. However, you cannot remove an address prefix if it contains any subnets.

Adding an Address Prefix

1. Navigate to your Virtual Network resource in the Azure portal.
2. Under Settings, select Address space.
3. Click on + Add address space.
4. Enter the new CIDR block for your address prefix.
5. Click Save.

Removing an Address Prefix

Important: You can only remove an address prefix if it does not contain any subnets. If you need to remove a prefix that has subnets, you must first delete those subnets or move them to a different address space.

1. Navigate to your Virtual Network resource in the Azure portal.
2. Under Settings, select Address space.
3. Click the 'X' icon next to the address prefix you wish to remove.
4. Confirm the removal.

Example Scenario

Let's say you are designing a network for a web application. You might start with a VNet address space of 10.1.0.0/16. This gives you ample room to create subnets for your web servers, application servers, and databases. If you later need to connect this VNet to another VNet with an address space of 192.168.0.0/20, there would be no conflict.

Consider this scenario: you have a VNet with address space 10.1.0.0/16 and a subnet 10.1.1.0/24. If you want to add another VNet using 10.1.0.0/24, this will cause an overlap with the existing subnet, and you'll need to adjust one of the address spaces.

Best Practices

• Use private IP ranges: Always use RFC 1918 private IP address ranges for your VNets.
• Document your IP addressing scheme: Maintain clear documentation of your VNet and subnet IP ranges for easy reference and troubleshooting.
• Start with a sufficiently large address space: Avoid the need for future re-architecting by planning for growth.
• Review and validate connections: Before establishing VNet peering or VPN connections, always verify that the address spaces of the connected networks do not overlap.

By carefully planning and managing your Azure Virtual Network address space, you can build a robust, scalable, and secure cloud network foundation.