List<string> list = new List<string>();
//Начало
byte[] start = System.Net.IPAddress.Parse("170.66.0.0").GetAddressBytes();
Array.Reverse(start);
var x_start = BitConverter.ToUInt32(start, 0); // int
// Конец
byte[] end = System.Net.IPAddress.Parse("170.66.255.255").GetAddressBytes();
Array.Reverse(end);
var x_end = BitConverter.ToUInt32(end, 0);
for (var i = x_start; i <= x_end; i++) {
byte[] ip_bytes = BitConverter.GetBytes(i);
Array.Reverse(ip_bytes);
var newIp = new System.Net.IPAddress(ip_bytes);
list.Add(newIp.MapToIPv4().ToString());
}
return string.Join(Environment.NewLine, list); // Результат