I am in need to find out how I can find out if a provided subnet mask example (255.255.192.0) is a valid subnet mask and return true if it is valid and false if not, I already am checking if the value is over 255. A wrong subnet would be (255.64.0.0)
It makes a lot of sense in binary (11111111.01000000.00000000.00000000) A subnet can not stop having 1's, and then start having them again. My current idea involves using bitshift, but am unsure how to do it.
I am not using any libaries and am not allowed for this project
The code I am using goes something like
Console.WriteLine("Enter a subnet mask");
input = Console.ReadLine(); //Enters example of 255.64.0.0 which is invalid
Thanks in advance, ask questions if needed
You can try this way:
using System;
using System.Runtime.InteropServices;
namespace Example
{
public class Program
{
[StructLayout(LayoutKind.Explicit)]
public struct byte_array
{
[FieldOffset(0)]
public byte byte0;
[FieldOffset(1)]
public byte byte1;
[FieldOffset(2)]
public byte byte2;
[FieldOffset(3)]
public byte byte3;
[FieldOffset(0)]
public UInt32 Addr;
}
public static void Main(string[] args)
{
byte_array b_array = new byte_array();
int i;
b_array.byte3 = 255;
b_array.byte2 = 64;
b_array.byte1 = 0;
b_array.byte0 = 0;
Console.WriteLine(String.Format("{0:X4}", b_array.Addr));
for(i = 31; i >= 0; i--)
if(((1 << i) & b_array.Addr) == 0)
break;
for(; i >= 0; i--)
if(((1 << i) & b_array.Addr) != 0)
{
Console.WriteLine("Bad Mask!");
break;
}
}
}
}
I looked for a library method, but couldn't find one. Here's how I would write it for IPv4 addresses only;
public static bool IsValidMask(string mask)
{
// 1) convert the address to an int32
if (!IPAddress.TryParse(mask, out var addr))
return false;
var byteVal = addr.GetAddressBytes();
if (byteVal.Length != 4)
return false;
var intVal = BitConverter.ToInt32(byteVal);
intVal = IPAddress.NetworkToHostOrder(intVal);
// A valid mask should start with ones, and end with zeros 0b111...111000...000
// 2) XOR to flip all the bits (0b000...000111...111)
var uintVal = (uint)intVal ^ uint.MaxValue;
// 3) Add 1, causing all those 1's to become 0's. (0b000...001000...000)
// An invalid address will have leading 1's that are untouched by this step. (0b101...001000...000)
var addOne = uintVal + 1;
// 4) AND the two values together, to detect if any leading 1's remain
var ret = addOne & uintVal;
return ret == 0;
}
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