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When you set up a virtual network, you specify the topology of the virtual network, including the available address spaces and subnets. If the virtual network is to be connected to other virtual networks, you must select address ranges that are not overlapping. This is the range of addresses that the VMs and services in your network can use. These will be private and cannot be accessed from the public Internet. This used to be true only for the unrouteable IP addresses such as 10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16, but now Azure will treat any address range as part of the private VNet IP address space that is only reachable within the VNet, within interconnected VNets, and from your on-premises location.

CIDR specifies an address range using a combination of an IP address and its associated network mask. CIDR notation uses this format: xxx.xxx.xxx.xxx/n, where n is the number of leftmost “1” bits in the mask. For example, 192.168.12.0/23 applies the network mask 255.255.254.0 to the 192.168 network, starting at 192.168.12.0. This notation therefore represents the address range 192.168.12.0–192.168.13.255. Fortunately, the Azure portal displays this information on the screen after you type the CIDR into one of the address space fields so you don’t have to do bit-wise math to figure it out!
10.0.0.0/8 gives you a usable address range of 10.0.0.0–10.255.255.255. You can certainly use this, but if 10.0.0.0 is being used elsewhere in the network, either on-premises or within Azure, you want to be sure you have no overlap. One way to do this is to specify an address space that is smaller but still has the capacity to hold everything you want to put in it. For example, if you are just going to put a handful of VMs in your virtual network, you could use 10.0.0.0/27, which gives you a usable address range of 10.0.0.0–10.0.0.31.

If you work within an organization in which someone else is responsible for the internal networks, you should confer with that person before selecting your address space to make sure there is no overlap and to let them know what space you want to use so they don’t try to use the same range of IP addresses.

Source of Information : Microsoft Azure Essentials Fundamentals of Azure Second Edition