When you set up a virtual network, you define the internal address space in Classless Interdomain Routing (CIDR) format. This address space needs to be unique within your subscription and any other networks that you connect to.
Let's assume you choose an address space of 10.0.0.0/24 for your first virtual network. The addresses defined in this address space range from 10.0.0.1 to 10.0.0.254. You then create a second virtual network and choose an address space of 10.0.0.0/8. The addresses in this address space range from 10.0.0.1 to 10.255.255.254. Some of the addresses overlap and can't be used for the two virtual networks.
But you can use 10.0.0.0/16, with addresses that range from 10.0.0.1 to 10.0.255.254, and 10.1.0.0/16, with addresses that range from 10.1.0.1 to 10.1.255.254. You can assign these address spaces to your virtual networks because there's no address overlap.
