產生 IP v6 的 mask byte array

int length = 121; /* total 128 */

var mask = new byte[16];

for (int i = 0; i < 16; i++)
{
    mask[i] = 0xff;
    if (length > -8) { length -= 8; }
    if (length < 0) { mask[i] = (byte)(mask[i] << -length); }
    /* 當 length 出現負值時代表需要進行位移 */
}

BitConverter.ToString(mask).Dump();
/* FF-FF-FF-FF-FF-FF-FF-FF-FF-FF-FF-FF-FF-FF-FF-80 */