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CDP1802 wrote:

The machine code for this would be about 20 - 30 bytes long

[Test]
public voi

public enum IP_TYPE
{
	IPv4
,	IPv6
}

public String convertCIDR2DottedDecimal(int nCIDR, IP_TYPE eType)
{
	// Build Binary String
	int nBits = (eType == IP_TYPE.IPv4) ? 32 : 128;
	String sBinary = "";

	for(int nIdx = nCIDR - 1; nIdx >= 0; nIdx--)
		sBinary += "1";

	for(int nIdx = nBits - nCIDR; nIdx >= 0; nIdx--)
		sBinary += "0";

	// Split Binary String to Octets
	int nOctets = (eType == IP_TYPE.IPv4) ? 4 : 16;
	String[] aOctets = new String[nOctets];

	for(int nIdx = 0; nIdx < nOctets; nIdx++)
	{
		String sOctet = "";
		for(int nOctetIdx = 0; nOctetIdx < 8; nOctetIdx++)
		{
			sOctet += sBinary[nOctetIdx];
		}

		sBinary = sBinary.Substring(8);

		// Convert each Octet to Decimal
		aOctets[nIdx] = "" + Convert.ToInt32(aOctets[nIdx], 2);
	}

	return String.Join(".", aOctets);
}

string subnetMaskIPv4(int bits)
{
    uint max = 4294967295;
    uint mask = max << (32 - bits);
    byte[] maskBytes = BitConverter.GetBytes(mask);
    Array.Reverse(maskBytes);
    return String.Join(".", maskBytes);
}