In C#, I have a subnet bits and need to create a Subnet Mask. How might I do this?

Question

I have a task to complete in C#. I have a Subnet Name: 192.168.10.0/24

I need to find the subnet mask, which would be, in this case, 255.255.255.0.

However, I need to be able to do this in C# WITHOUT the use of the System.Net library (the system I am programming in does not have access to this library).

It seems like the process should be something like:

1) Split the Subnet Name into Number and Bits.

2) Shove the Bits into this that I have found on SO (thanks to Converting subnet mask "/" notation to Cisco 0.0.0.0 standard ):

var cidr = 24; // e.g., "/24" 
var zeroBits = 32 - cidr; // the number of zero bits 
var result = uint.MaxValue; // all ones 

// Shift "cidr" and subtract one to create "cidr" one bits; 
//  then move them left the number of zero bits. 
result &= (uint)((((ulong)0x1 << cidr) - 1) << zeroBits); 

// Note that the result is in host order, so we'd have to convert 
//  like this before passing to an IPAddress constructor 
result = (uint)IPAddress.HostToNetworkOrder((int)result); 

However, the problem that I have is that I do not have access to the library that contains the IPAddress.HostToNetworkOrder command in the system that I am working. Also, my C# is pretty poor. Does anyone have the C# knowledge to help?

Answer 1

You could replace that method with the following:

static void ToNetworkByteOrder(ref uint n) {
    if(BitConverter.IsLittleEndian) {
        // need to flip it
        n = (
            (n << 24)
            |
            ((n & 0xff00) << 8)
            |
            ((n & 0xff0000) >> 8)
            |
            (n >> 24)
        );
    }
}

Answer 2

Here's a simpler way to get the mask:

int mask = -1 << (32 - cidr);

You don't need the Net assembly to get the bytes in the right order, you can use the BitConverter class:

if (BitConverter.IsLittleEndian) {
  byte[] parts = BitConverter.GetBytes(mask);
  Array.Reverse(parts);
  mask = BitConverter.ToInt32(parts, 0);
}