take the bits for host and do

2 to the power to host bits - 2


Memorize Table
----------------
2  2 = 4       |
----------------
2 3 = 8        |
----------------
2 4 = 16       |
----------------
2 5 = 32       |
----------------
2 6 = 64       |
----------------
2 7 = 128      |
----------------
2 8 = 256      |
----------------
2 9 = 512      |
----------------
2 10 = 1024    |
----------------
2 11 = 2048    |
----------------
2 12 = 4096    |
----------------
2 13 = 8192    |
----------------
2 14 = 16384   |
----------------
2 15 = 32768   |
----------------
2 16 = 65536   |
----------------

    Source: geocities.com/xvset