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Virtual Networks: Addressing

This article delves into the crucial aspect of addressing within Azure Virtual Networks (VNet). Proper IP addressing is fundamental for enabling communication between your Azure resources and between your Azure VNet and on-premises networks.

Understanding IP Addressing in Azure VNets

An Azure VNet provides a representation of your private network in the cloud. You define a private IP address space for your VNet, which can include one or more Classless Inter-Domain Routing (CIDR) blocks. These blocks dictate the range of IP addresses that can be assigned to resources within that VNet.

Key Concepts:

• Address Space: The range of IP addresses allocated to your VNet, specified using CIDR notation (e.g., 10.0.0.0/16).
• Subnets: VNets are divided into subnets, each assigned a subset of the VNet's address space. This segmentation helps in organizing resources and applying network policies.
• Private IP Addresses: Resources within a VNet are assigned private IP addresses from the VNet's address space. These addresses are not routable on the public internet.
• Public IP Addresses: Optionally, resources can be assigned public IP addresses for direct internet access.

CIDR Notation Explained

CIDR notation is a method for representing IP addresses and their associated network prefixes. It's typically written as an IP address followed by a slash and a number, such as 192.168.1.0/24. The number after the slash indicates the number of bits used for the network portion of the IP address. A /24 means the first 24 bits define the network, leaving 8 bits for host addresses, allowing for 256 addresses (though two are reserved).

Address Allocation and Reservations

Azure reserves the first four and the last IP address of every subnet for its own use. This means that for a subnet with a /24 prefix (256 addresses), only 253 are available for your resources.

• .0: Network address
• .1: Default gateway
• .2: Azure DNS
• .3: Azure DNS
• .255: Broadcast address

Designing Your VNet Address Space

When designing your VNet address space, consider the following:

• Avoid Overlap: Ensure that your VNet address space does not overlap with your on-premises network address space if you plan to establish a VPN or ExpressRoute connection. Overlapping IP addresses will lead to routing conflicts and connectivity issues.
• Plan for Growth: Allocate a sufficiently large address space to accommodate future expansion and the addition of new resources. It's easier to start with a larger block than to reconfigure an existing VNet later.
• Segmentation with Subnets: Use subnets to logically group resources with similar security or routing requirements. This granular control enhances network management and security posture.

Example Scenario

Consider a VNet with the address space 10.1.0.0/16. This provides a total of 65,536 IP addresses. You could then define subnets within this space:

Best Practice: For maximum flexibility, start with a larger VNet address space than you initially think you'll need. Overlapping IP address spaces between on-premises and cloud networks is a common pitfall.

Next Steps

Understanding VNet addressing is the first step. The next logical step is to learn about configuring subnets and managing IP address assignments for your Azure resources.

• Configuring Subnets in Azure Virtual Networks
• Managing IP Addresses for Azure Resources