Understand Azure Virtual Network, SubNet, CIDR Notation, Resource Group

Santosh Gaikwad

Santosh Gaikwad

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>> Azure

Azure Virtual Network

When a new VM is provisioned on Azure,  physical access to the hosting machine is never given,  Instead, need to operate the machine through remote connections such as remote desktop or Secure Shell (SSH).

This is made possible by the networking infrastructure provided by Azure. VMs deployed on a virtual network can communicate with one another just as if they are on an on-premises local area network (LAN).

Azure Virtual Network connection with on-premises network
Azure Virtual Network connection with on-premises network

The Azure Virtual Network service enables to securely connect Azure resources to each other with virtual networks (VNets).

A VNet is a representation of own network in the cloud. A VNet is a logical isolation of the Azure cloud dedicated to a subscription.

VNets can also be connected to on-premises network. The following picture shows some of the capabilities of the Azure Virtual Network.

There are several different ways to create a new virtual network on Azure, including using the Azure management portal, Azure PowerShell, and xplat-cli.

Creating a cloud-only virtual network

1. Sign in to the management portal (https://portal.azure.com).
2. Select New -> Networking -> Virtual Network -> Create as shown in figure below

Create Azure Virtual Network
Create Azure Virtual Network
Azure Virtual Network Details
Azure Virtual Network Details

3. On the Virtual network blade, type the Name of the VNet, and then click Address space.

Configure your address space settings for the VNet and its first Subnet, then click OK.

The figure below shows the CIDR block settings for our scenario.

How CIDR notations work? (Subnet Address Range)

CIDR (Classless Inter domain routing) notation is a compact representation of an IP address and its associated routing prefix.

The notation is constructed from an IP address, a slash (‘/’) character, and a decimal number. The number is the count of leading 1 bits in the routing mask, traditionally called the network mask.

The IP address is expressed according to the standards of IPv4 or IPv6.

The address may denote a single, distinct interface address or the beginning address of an entire network. The maximum size of the network is given by the number of addresses that are possible with the remaining, least-significant bits below the prefix.

The aggregation of these bits is often called the host identifier. lets consider following example.

The IPv4block 10.0.0.0/24 represents the 256 IPv4 addresses from 10.0.0.0 to 10.0.0.255.

Before the implementation of CIDR, IPv4 networks were represented by the starting address and the subnet mask, both written in dot-decimal notation. Thus, 10.0.0.0/24 was often written as 10.0.0.0/255.0.0.0.

The number of addresses of a subnet may be calculated as 2address size − prefix size, in which the address size is 128 for IPv6 and 32 for IPv4. For example, in IPv4, the prefix size /29 gives: 232 − 29 = 23 = 8 addresses.

4. Click Resource Group and select a resource group to add the VNet to, or click create new resource group to add the VNet to a new resource group. The figure below shows the resource group settings for a new resource group called SCIResourceGroupLab1.

What is Resource Group?
Resource group a container that holds related resources for an Azure solution. The resource group can include all the resources for the solution, or only those resources that you want to manage as a group.

You decide how you want to allocate resources to resource groups based on what makes the most sense for your organization.

5. Create Subnet
Click Subnets -> Add. Provide name, address range (CIDR block) for Subnet, and then click OK.

VNet SubNet Representation
VNet SubNet Representation

What is subnet?
VNet can be segmented into multiple subnets. Sub netting allows to create multiple logical networks that exist within a single network.

e.g. Create subnets for logical separation of resources e.g. FrontEnd subnet, using 10.0.0.32/28 as its CIDR block, this subnet will have 2^(32-28) = 2^4 = 16 IPs available, with IPs ranging from 10.0.0.32 to 10.0.0.47

BackEnd, using 10.0.0.48/28 as its CIDR block, this subnet will have 2^(32-28) = 2^4 = 16 IPs available, with IPs ranging from 10.0.0.48 to 10.0.0.63

 

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