public static class SubnetMask
    {
        public static readonly IPAddress ClassA = IPAddress.Parse("255.0.0.0");
        public static readonly IPAddress ClassB = IPAddress.Parse("255.255.0.0");
        public static readonly IPAddress ClassC = IPAddress.Parse("255.255.255.0");

        public static IPAddress CreateByHostBitLength(int hostpartLength)
        {
            int hostPartLength = hostpartLength;
            int netPartLength = 32 - hostPartLength;

            if (netPartLength < 2)
                throw new ArgumentException("Number of hosts is to large for IPv4");

            Byte[] binaryMask = new byte[4];

            for (int i = 0; i < 4; i++)
            {
                if (i * 8 + 8 <= netPartLength)
                    binaryMask[i] = (byte)255;
                else if (i * 8 > netPartLength)
                    binaryMask[i] = (byte)0;
                else
                {
                    int oneLength = netPartLength - i * 8;
                    string binaryDigit =
                        String.Empty.PadLeft(oneLength, '1').PadRight(8, '0');
                    binaryMask[i] = Convert.ToByte(binaryDigit, 2);
                }
            }
            return new IPAddress(binaryMask);
        }

        public static IPAddress CreateByNetBitLength(int netpartLength)
        {
            int hostPartLength = 32 - netpartLength;
            return CreateByHostBitLength(hostPartLength);
        }

        public static IPAddress CreateByHostNumber(int numberOfHosts)
        {
            int maxNumber = numberOfHosts + 1;

            string b = Convert.ToString(maxNumber, 2);

            return CreateByHostBitLength(b.Length);
        }
    }

    public static class IPAddressExtensions
    {
        public static IPAddress GetBroadcastAddress(this IPAddress address, IPAddress subnetMask)
        {
            byte[] ipAdressBytes = address.GetAddressBytes();
            byte[] subnetMaskBytes = subnetMask.GetAddressBytes();

            if (ipAdressBytes.Length != subnetMaskBytes.Length)
                throw new ArgumentException("Lengths of IP address and subnet mask do not match.");

            byte[] broadcastAddress = new byte[ipAdressBytes.Length];
            for (int i = 0; i < broadcastAddress.Length; i++)
            {
                broadcastAddress[i] = (byte)(ipAdressBytes[i] | (subnetMaskBytes[i] ^ 255));
            }
            return new IPAddress(broadcastAddress);
        }

        public static IPAddress GetNetworkAddress(this IPAddress address, IPAddress subnetMask)
        {
            byte[] ipAdressBytes = address.GetAddressBytes();
            byte[] subnetMaskBytes = subnetMask.GetAddressBytes();

            if (ipAdressBytes.Length != subnetMaskBytes.Length)
                throw new ArgumentException("Lengths of IP address and subnet mask do not match.");

            byte[] broadcastAddress = new byte[ipAdressBytes.Length];
            for (int i = 0; i < broadcastAddress.Length; i++)
            {
                broadcastAddress[i] = (byte)(ipAdressBytes[i] & (subnetMaskBytes[i]));
            }
            return new IPAddress(broadcastAddress);
        }

        public static bool IsInSameSubnet(this IPAddress address2, IPAddress address, IPAddress subnetMask)
        {
            IPAddress network1 = address.GetNetworkAddress(subnetMask);
            IPAddress network2 = address2.GetNetworkAddress(subnetMask);

            return network1.Equals(network2);
        }

    }

private void Form1_Load(object sender, EventArgs e)
        {
            string hostName = Dns.GetHostName();
            string myIP = Dns.GetHostByName(hostName).AddressList[0].ToString();
            lblip.Text = myIP;

            lblmac.Text = DeviceInfo.GetMacAddress()[0].ToString();
            **lblmask.Text = SubnetMask.CreateByHostBitLength();**esta es la que no sale
        }