Virtual Network can have multiple Address Spaces, additional address spaces can be added during or after the creation of Virtual Network by going to Address Space option as shown in the below screenshot.
Subnets option in the settings will help you to add additional subnet across multiple address spaces that are part of Virtual Network. Subnet spanning across multiple address spaces of Virtual Network will communicate with each other. Azure creates those routes to enable communication.
Ex: In one of the above screenshots we got a Virtual network with address spaces 10.0.0.0/16 and 172.16.0.0/16, Subnets using these address spaces will communicate with each other.
To create additional Subnets using the highlighted Subnet creation option, and provide name and IP address range. The range of IP address you can provide here can subset of the entire address spaces of the Virtual Network.
Azure reserves 5 IP addresses for each Subnet for its internal usage purpose. In the below screenshot for Subnet1 IP address range 10.0.0.0/24 which provides 256 IPs, however, the available IP address are 251.
For 10.0.0.0/24 Subnet following are the IP address that are reserved.
DNS Servers Option will help to update your Organization DNS servers details, When you create Virtual Network by default it picks Azure DNS Servers, use the Custom option to update DNS servers details.
f DNS servers details are updated after the Virtual Machines are created in the Virtual Network, virtual machines require a reboot to get new DNS IP addresses.