UPDATED ANSWER:
Just try with this...
var bytes = IPAddress.Parse("192.168.1.33").GetAddressBytes();
// set the value here
bytes[3] = 100;
IPAddress ipAddress = new IPAddress(bytes);
OR
var bytes = IPAddress.Parse("192.168.1.33").GetAddressBytes();
// set the value here
bytes[3] = 100;
System.Text.StringBuilder ipAddress = new System.Text.StringBuilder();
foreach (byte b in bytes)
{
ipAddress.AppendFormat("{0}.", b);
}
string ipAddress1 = ipAddress.ToString();
ipAddress1 = ipAddress1.TrimEnd('.');
var newIp = IPAddress.Parse(ipAddress1);