Private Address Spaces

The RFC 1918 standard defines three private address spaces that are never used for addressing on the Internet. Administrators use these ranges behind Network Address Translation (NAT) devices to ensure unique addresses used within intranets never prevent communication with Internet servers. These three address spaces are the only ones that are supported within an Azure VNet. The address spaces are:·

10.0.0.0/8. This address space includes all addresses from 10.0.0.1 to 10.0.0.255.     
172.16.0.0/12. This address space includes all addresses from 172.16.0.1 to 172.31.255.255.      
192.168.0.0/16. This address space includes all addresses from 192.168.0.1 to 192.168.255.255.

When you specify an address space for a VNet, you usually specify a much smaller range within one of the private address spaces. For example, if you specified the address space 10.1.1.0/24, that means that all addresses from 10.1.1.1 to 10.1.1.255 should be routed into your VNet.

In a cloud-only virtual network, you can specify any address range from the RFC 1918 private spaces. However, if you connect to a VNet with a VPN or ExpressRoute, you must ensure that the address space is unique and does not overlap any of the ranges that are already in use on-premises or in other VNets.