VNet is similar to a traditional network that you'd operate in your own data center, but brings with it additional benefits of Azure's infrastructure such as scale, availability, and isolation.
Subnets help us get logical divisions within our network which help us increase performance, improve security and make it easier to manage the network.
A virtual network can be segmented into one or more subnets. Each and every subnet contains a range of IP addresses that fall within the virtual network address space.
This range should be unique within the address space and not overlap with other subnet address ranges. The address space should be specified using CIDR.
If a subnet's traffic is routed to an internet gateway, the subnet is known as a public subnet.