Get netmaske from CIDR

Question

How do I get the netmask of any CIDR, e.g.
CIDR=10 > 255.255.0.0
CIDR=24 > 255.255.255.0
CIDR=25 > 255.255.255.128
CIDR=31 > 255.255.255.254

Answer 1

This will create a byte[4] array with the four netmask octet. You assign your CIDR to the variable CIDRInt

int CIDRInt = 26;
int index = 0;
byte[] CIDROctet = new byte[4] { 0, 0, 0, 0 };

for (int i = 1; i <= 32; i++)
{
    if (i > CIDRInt) { break; }
    if (index == 8) { index = 0; }

    if (i > 24)
    {
        CIDROctet[3] |= (byte)(1 << (7 - index)); //index from 7 til -1
        index++;
    }
    else if (i > 16)
    {
        CIDROctet[2] |= (byte)(1 << (7 - index));
        index++;
    }
    else if (i > 8)
    {
        CIDROctet[1] |= (byte)(1 << (7 - index)); 
        index++;
    }
    else
    {
        CIDROctet[0] |= (byte)(1 << (7 - index));
        index++;
    }
}

string CIDRStr = CIDROctet[0] + "." + CIDROctet[1] + "." + CIDROctet[2] + "." + CIDROctet[3];

UPDATE:
I like the idea of calculating the netmask as above, but I ended up creating a "low-tech" class with manual static mappings of CIDR<>Netmask for assumable better performance and two-way-convertion. The class also return the netmask-class (A, B or C) and the amount of covered IP addresses of each netmask. The class is available at github.com