Q. I've noticed that when I create a VM from the new Azure portal a virtual network is automatically created. Why?
A. VMs in Azure, created using the previous portal, were created in a cloud service which could optionally be placed in a virtual network. If VMs were not placed in a virtual network then the VMs in the cloud service could communicate with each other but not with VMs in other cloud services. You also had no control over the IP address space used. The direction going forward is that VMs always exist within a virtual network. This means that, whenever you create a new VM in the new Azure portal, if you do not manually specify a virtual network then one will be created for you with the same name as the VM. My recommendation is to manually create a virtual network in the regions where you create VMs and manually specify that virtual network during the VM creation.
Note that if a virtual network already exists within the resource group that the VM is being created within then that virtual network will be used automatically.
The address space used by the virtual network that is automatically created cannot be easily determined. The portal will look at existing virtual networks already in the subscription by performing a scan and will use the first address space with a /16 gap. All three of the private IP classes from RFC 1918 can be used.