Generate Random Number in ASP.NET C#

public string GetRandonNumber()
{
    StringBuilder builder = new StringBuilder();
    builder.Append(RandomString(4, false));  // for first 4 number are character
    builder.Append(RandomNumber(1000, 9999)); // for 2nd 4 number are number
    builder.Append(RandomString(2, true)); // for last 2 number are character
    string str= builder.ToString();
    return str;
}
public string RandomString(int size, bool lowerCase)
{

    StringBuilder builder = new StringBuilder();
    Random random = new Random();
    char ch;
    for (int i = 0; i < size; i++)
    {
        ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65)));
        builder.Append(ch);
    }
    if (lowerCase)
    {
        return builder.ToString().ToLower();
    }
    return builder.ToString();
}
private int RandomNumber(int min, int max)
{
    Random random = new Random();
    return random.Next(min, max);
}