Can I create a network in Azure with the same subnet as the onpremise LAN?

Question

Hi,

I need to create an Express Route to establish communication between my Azure and Datacenter Equinix.

I talked to an Equinix specialist and assured me that I can create a vNet with the same subnet as my LAN network in the onpremise environment.

In conversation with a friend from Azure Microsoft Network he confirmed that this is not possible, as this will cause the so-called overlap.

Does using an Express Route have any benefits for using the same subnet and that is why the Equinix specialist informed me that we can?

Thank you.

Answer 1

Hello @Salves ,

You cannot create a virtual network in Azure with the same subnet/address range as your onpremise LAN, if you want to connect your on-premise network to this particular virtual network using VPN or ExpressRoute.
Please refer : https://learn.microsoft.com/en-us/azure/virtual-network/virtual-networks-overview#best-practices

Creating an Azure Vnet with same address range as your on-premise network and connecting them via ExpressRoute will cause routing problems. Hence when using a virtual network as part of a cross-premises architecture, be sure to coordinate with your on-premises network administrator to carve out an IP address range that you can use specifically for this virtual network. If a duplicate address range exists on both sides of the VPN/ExpressRoute connection, traffic will route in an unexpected way.

Please refer : https://learn.microsoft.com/en-us/azure/cloud-adoption-framework/ready/enterprise-scale/network-topology-and-connectivity

Kindly let us know if the above helps or you need further assistance on this issue.

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Answer 2

I believe yes you can but you need to use the new feature from Azure VPN NAT ,here you go the link :

https://learn.microsoft.com/en-us/azure/vpn-gateway/nat-howto

Answer 3

In many cases its needed configure same network ranges in the both sides (on-premises / azure)
In many cases its needed configure same network ranges in the both sides (azure / other locations with ipsec s2s tunnel)
Many times we have Production environments in azure and staging/dev environments on-premises or in another cloud with s2s connections and we need preserve same private ips.

NAT over connections with Network Gateway: https://learn.microsoft.com/en-us/azure/vpn-gateway/nat-overview
"NAT is supported on the the following SKUs: VpnGw2~5, VpnGw2AZ~5AZ."
"NAT is supported on IPsec cross-premises connections only. VNet-to-VNet connections or P2S connections are not supported."