Hello @Dalbir Singh ,
Welcome to Microsoft Q&A Platform. Thank you for reaching out & hope you are doing well.
I understand that you would like to know more about Subnet extension in Azure.
Extended network for Azure enables you to stretch an on-premises subnet into Azure to let on-premises virtual machines keep their original on-premises private IP addresses when migrating to Azure.
Extended network for Azure should only be used for machines that cannot have their IP address changed when migrating to Azure. It is always better to change the IP address and connect it to a subnet that wholly exists in Azure, if that is an option.
This subnet extension option in Azure is only for scenarios when you would like to connect your on-premises network to Azure but with overlapping IP addresses.
By default, Azure doesn't allow overlapping IP addresses between Azure and on-premises connection.
Does this need a vpn on prem? Is this built of a Site to Site link?
Yes, to connect Azure with your on-premises network, you either need a site-to-site VPN or Azure ExpressRoute connection.
The Article from Lee below used pfsense.
Yes, to connect to Azure via Site-to-Site VPN, you would need a VPN device in your on-premises with which you would form a VPN tunnel.
Refer: https://video2.skills-academy.com/en-us/azure/vpn-gateway/vpn-gateway-about-vpn-devices
The on prem solution requires two HyperV switches - i'm not sure why a switch is being created. I don't understand the network topology in these situations.
I'm not sure which Azure certification material you are following but as per the below extended network for Azure document,
You need to create a Windows Server 2019 or 2022 VM on any hypervisor that supports nested virtualization. This is the on-premises virtual appliance. We recommend that you create this as a highly available VM in a cluster. Connect a virtual network adapter to the routable subnet and a second virtual network adapter to the extended subnet.
Hyper-V is a hypervisor that allows you to create and manage virtual machines. Hyper-V specifically provides hardware virtualization. That means each virtual machine runs on virtual hardware.
Refer: https://video2.skills-academy.com/en-us/virtualization/hyper-v-on-windows/about/
So, basically you are creating a Virtual machine on your on-premises machine to act as a virtual appliance for proper traffic routing.
What does Gateway subnet refer to?
GatewaySubnet is the dedicated subnet in an Azure Virtual Network where you can deploy an Azure VPN gateway.
Before you create a VPN gateway, you must create a gateway subnet. The gateway subnet contains the IP addresses that the virtual network gateway VMs and services use. When you create your virtual network gateway, gateway VMs are deployed to the gateway subnet and configured with the required VPN gateway settings.
What does NVA refer to in this instance - is this a term
Yes, NVA refers to Network Virtual Appliance.
An NVA is typically used to control the flow of traffic between network segments classified with different security levels, for example between a De-Militarized Zone (DMZ) Virtual Network and the public Internet.
https://video2.skills-academy.com/en-us/azure/architecture/networking/guide/nva-ha
Kindly let us know if the above helps or you need further assistance on this issue.
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