Deploying virtual subnets
There seems to be no way of deploying virtual subnets in the portal apart from creating them when deploying virtual networks.
It would be sensible to be able to deploy virtual subnets from the portal.
In my thinking the virtual subnets are separate object which I want to manage in the same way as the virtual networks.
anand malla commented
subnet is a part of virtual network(VNET).
To create subnet we must mention vnet first. We assign some range of IP addresses for Virtual Network(VNET), among them we select and assign one by one IP Addresses for every one virtual machine to make them connect in one network.