What is a Virtual Network?

A virtual network (VNet) is a logical representation of a traditional network that you would find in your own data center, but with the benefits of the physical infrastructure. VNets are deployed within a cloud provider's infrastructure, offering isolated and secure private network spaces for your cloud resources.

Key characteristics of a virtual network include:

• Isolation: Each VNet is isolated from other VNets, providing security and preventing unintended network traffic between different environments.
• Segmentation: VNets can be segmented into subnets, allowing for granular control over network traffic flow within the VNet.
• Connectivity: Resources within a VNet can communicate with each other, and with the internet, securely. VNets can also be connected to other VNets and on-premises networks.
• IP Addressing: VNets use private IP address spaces, which are not routable on the public internet, ensuring security.

Core Components of a Virtual Network

Several components work together to form a functional virtual network:

Subnets

A subnet is a range of IP addresses within a VNet. Dividing a VNet into subnets allows you to segment the network for security and organizational purposes. Resources are deployed into specific subnets.

IP Addresses

Virtual networks utilize both private and public IP addresses. Resources within a VNet are assigned private IP addresses. Public IP addresses are used to allow inbound and outbound internet connectivity for resources.

Network Security Groups (NSGs)

NSGs act as a distributed firewall, enabling you to define security rules that allow or deny network traffic to and from resources in a VNet. Rules can be based on source and destination IP addresses, ports, and protocols.

Route Tables

Route tables enable you to define custom routes to forward network traffic to particular destinations. This is crucial for scenarios like forcing traffic through a network virtual appliance (NVA) or directing traffic to an on-premises network.

Gateways

Network gateways facilitate connectivity. This can include VPN gateways for connecting to on-premises networks or ExpressRoute gateways for dedicated private connections. Virtual network gateways also enable VNet peering.

Virtual Network Use Cases

Virtual networks are fundamental to many cloud architectures:

• Isolating Workloads: Deploying different applications or tiers of an application into separate VNets or subnets for enhanced security.
• Hybrid Cloud Connectivity: Securely connecting your cloud virtual networks to your on-premises data centers.
• Disaster Recovery: Establishing redundant network environments in different regions.
• Network Segmentation: Creating multiple subnets for different purposes, such as a front-end subnet, a back-end subnet, and a database subnet.

Advanced Concepts

VNet Peering

VNet peering allows you to connect two Azure virtual networks together. Once peered, networks can communicate with each other as if they were one network. Traffic between peered VNets is routed through the Microsoft backbone network; it does not traverse the public internet.

Network Virtual Appliances (NVAs)

NVAs are virtual machines that function as network appliances, such as firewalls, load balancers, or WAN accelerators. They can be deployed within a VNet to provide advanced network services.

# Example: Creating a basic virtual network with a subnet (Conceptual Azure CLI)
az network vnet create \
    --resource-group MyResourceGroup \
    --name MyVNet \
    --address-prefix 10.0.0.0/16

az network vnet subnet create \
    --resource-group MyResourceGroup \
    --vnet-name MyVNet \
    --name MySubnet \
    --address-prefix 10.0.1.0/24