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Azure virtual networks enable Azure resources, such as VMs, web apps, and databases, to communicate with each other, with users on the internet, and with your on-premises client computers. You can think of an Azure network as a set of resources that links other Azure resources.
Isolation and segmentation
Virtual Network allows you to create multiple isolated virtual networks. When you set up a virtual network, you define a private IP address space by using either public or private IP address ranges. You can divide that IP address space into subnets and allocate part of the defined address space to each named subnet.
For name resolution, you can use the name resolution service that’s built in to Azure. You also can configure the virtual network to use either an internal or an external DNS server.
A VM in Azure can connect to the internet by default. You can enable incoming connections from the internet by defining a public IP address or a public load balancer. For VM management, you can connect via the Azure CLI, Remote Desktop Protocol, or Secure Shell.
Communicate between Azure resources
You’ll want to enable Azure resources to communicate securely with each other. You can do that in one of two ways:
Virtual networks: Virtual networks can connect not only VMs but other Azure resources, such as the App Service Environment for Power Apps, Azure Kubernetes Service, and Azure virtual machine scale sets.
Service endpoints: You can use service endpoints to connect to other Azure resource types, such as Azure SQL databases and storage accounts. This approach enables you to link multiple Azure resources to virtual networks to improve security and provide optimal routing between resources.
Communicate with on-premises resources
Azure virtual networks enable you to link resources together in your on-premises environment and within your Azure subscription. In effect, you can create a network that spans both your local and cloud environments. There are three mechanisms for you to achieve this connectivity:
Point-to-site virtual private networks: This approach is like a virtual private network (VPN) connection that a computer outside your organization makes back into your corporate network, except that it’s working in the opposite direction. In this case, the client computer initiates an encrypted VPN connection to Azure to connect that computer to the Azure virtual network.
Site-to-site virtual private networks: A site-to-site VPN links your on-premises VPN device or gateway to the Azure VPN gateway in a virtual network. In effect, the devices in Azure can appear as being on the local network. The connection is encrypted and works over the internet.
Azure ExpressRoute: For environments where you need greater bandwidth and even higher levels of security, Azure ExpressRoute is the best approach. ExpressRoute provides dedicated private connectivity to Azure that doesn’t travel over the internet.
Route network traffic
By default, Azure routes traffic between subnets on any connected virtual networks, on-premises networks, and the internet. You also can control routing and override those settings, as follows:
Route tables: A route table allows you to define rules about how traffic should be directed. You can create custom route tables that control how packets are routed between subnets.
Border Gateway Protocol: Border Gateway Protocol (BGP) works with Azure VPN gateways or ExpressRoute to propagate on-premises BGP routes to Azure virtual networks.
Filter network traffic
Azure virtual networks enable you to filter traffic between subnets by using the following approaches:
Network security groups: A network security group is an Azure resource that can contain multiple inbound and outbound security rules. You can define these rules to allow or block traffic, based on factors such as source and destination IP address, port, and protocol.
Network virtual appliances: A network virtual appliance is a specialized VM that can be compared to a hardened network appliance. A network virtual appliance carries out a particular network function, such as running a firewall or performing wide area network (WAN) optimization.
Virtual network peering
You can link virtual networks together by using virtual network peering. Peering enables resources in each virtual network to communicate with each other. These virtual networks can be in separate regions, which allows you to create a global interconnected network through Azure.
UDR is user-defined Routing or UDR is a significant update to Azure’s Virtual Networks as this allows network admins to control the routing tables between subnets within a subnet as well as between VNets thereby allowing for greater control over network traffic flow.