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Address space calculator for Azure gateway subnets

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A virtual network (VNet) in Azure infrastructure services that is connected to other networks must have a gateway subnet. The best practices for defining the gateway subnet are:

  • The prefix length of the gateway subnet can have a maximum prefix length of 29 (for example, 10.119.255.248/29), but the current recommendation is that you use a prefix length of 27 (for example, 10.119.255.224/27).
  • When defining the address space of the gateway subnet, use the last part of the VNet address space.

For the second recommendation, you can determine the address space of the gateway subnet by setting the bits used for the gateway subnet to 0 and the remaining bits in the VNet address space to 1. To quickly calculate the gateway subnet address space without having to convert to binary and back to decimal, you can use a console application written in C# or Python or with a PowerShell command block.

This article contains C#, Python, and PowerShell code blocks that calculate the gateway subnet address space based on the values of w.x.y.z/n for the VNet address prefix and the gateway subnet prefix length.

C# code block

Use this code block to create a console app in C#.

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Text; 
using System.Threading.Tasks; 
 
namespace ConsoleApplication1 
{ 
    class Program 
    { 
        static void Main(string[] args) 
        { 
            // Declare variables. 
            const long wOctet = 16777216;  
            const long xOctet = 65536; 
            const long yOctet = 256; 
            double D; 
            int w, x, y, z, vnetPrefLen, gwPrefLen; 
            long d, w2, x2, y2, z2; 
             
 
            // Get the five values needed from the keyboard. 
            Console.WriteLine("**************************************************************************"); 
            Console.WriteLine("*** Gateway subnet address space calculator for Azure virtual networks ***");             
            Console.WriteLine("**************************************************************************");  
            Console.WriteLine(); 
            Console.WriteLine("Please supply your virtual network address space in the form of w.x.y.z/n."); 
            Console.WriteLine(); 
            Console.WriteLine("w="); 
            w = Convert.ToInt32(Console.ReadLine()); 
            Console.WriteLine("x="); 
            x = Convert.ToInt32(Console.ReadLine()); 
            Console.WriteLine("y="); 
            y = Convert.ToInt32(Console.ReadLine()); 
            Console.WriteLine("z="); 
            z = Convert.ToInt32(Console.ReadLine()); 
            Console.WriteLine("n="); 
            vnetPrefLen = Convert.ToInt32(Console.ReadLine()); 
            Console.WriteLine(); 
            Console.WriteLine("Now supply the prefix length of your gateway subnet:"); 
            gwPrefLen = Convert.ToInt32(Console.ReadLine()); 
            Console.WriteLine(); 
 
            // Perform the calculation. 
            D = w * wOctet + x * xOctet + y * yOctet + z; 
            for (int i = vnetPrefLen + 1; i < gwPrefLen + 1; i++) 
            { 
                D = D + Math.Pow(2, 32 - i); 
            } 
            d = Convert.ToInt64(D); 
            w2 = d / wOctet; 
            x2 = (d - w2 * wOctet) / xOctet;  
            y2 = (d - w2 * wOctet - x2 * xOctet) / yOctet; 
            z2 = d - w2 * wOctet - x2 * xOctet - y2 * yOctet; 
             
            // Display the result.             
            Console.WriteLine("**************************************************************************");  
            Console.WriteLine("For the Azure virtual address space {0}.{1}.{2}.{3}/{4}", w, x, y, z, vnetPrefLen); 
            Console.WriteLine("and gateway prefix length of {0}, the gateway subnet address space is:", gwPrefLen); 
            Console.WriteLine(); 
            Console.WriteLine("{0}.{1}.{2}.{3}/{4}", w2, x2, y2, z2, gwPrefLen); 
            Console.WriteLine(); 
            Console.WriteLine("Press ENTER to quit."); 
            Console.ReadLine(); 
        } 
    } 
}

Python code block

Use this code block to create a console app in Python.

import math 
# Collect the values of w.x.y.z/n for your VNet address space and g, the prefix length of your gateway subnet 
print("**************************************************************************")  
print("*** Gateway subnet address space calculator for Azure virtual networks ***")  
print("**************************************************************************\n")   
print("Please supply your virtual network address space in the form of w.x.y.z/n.");  
w=int(input("w = ")) 
x=int(input("x = ")) 
y=int(input("y = ")) 
z=int(input("z = ")) 
n=int(input("n = ")) 
print("")  
g=int(input("Now supply the prefix length of your gateway subnet: ")) 
print("")  
 
# Calculate  
wOctet = 16777216  
xOctet = 65536  
yOctet = 256  
D = w * wOctet + x * xOctet + y * yOctet + z 
for index in range(n + 1,g + 1): 
    D += 2**(32 - index)  
 
w2 = math.floor(D / wOctet)  
x2 = math.floor((D - w2 * wOctet) / xOctet) 
y2 = math.floor((D - w2 * wOctet - x2 * xOctet) / yOctet) 
z2 = D - w2 * wOctet - x2 * xOctet - y2 * yOctet  
 
# Display the result  
gwAddrPref= "Your gateway address prefix is: " + str(w2) + "." + str(x2) + "." + str(y2) + "." + str(z2) + "/" + str(g)  
print(gwAddrPref)

PowerShell command block

Fill in the values and run the resulting command block in a PowerShell window or in the PowerShell Integrated Script Environment (ISE).

# Specify the values of w.x.y.z/n for your VNet address space and g, the prefix length of your gateway subnet: 
$w= 
$x= 
$y= 
$z= 
$n= 
$g= 
# Calculate 
$wOctet = 16777216 
$xOctet = 65536 
$yOctet = 256 
[long]$D = $w * $wOctet + $x * $xOctet + $y * $yOctet + $z; 
for ($i = $n + 1; $i -lt $g + 1; $i++) 
{ 
$D = $D + [math]::pow(2, 32 - $i) 
} 
$w2 = [math]::floor($D / $wOctet) 
$x2 = [math]::floor( ($D - $w2 * $wOctet) / $xOctet ) 
$y2 = [math]::floor( ($D - $w2 * $wOctet - $x2 * $xOctet) / $yOctet ) 
$z2 = $D - $w2 * $wOctet - $x2 * $xOctet - $y2 * $yOctet 
# Display the result 
$dx= [string]$w2 + "." + [string]$x2 + "." + [string]$y2 + "." + [string]$z2 + "/" + [string]$g 
Write-Host "Your gateway address prefix is: " $dx

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