Question

For a netmask of 255.255.128.0, tell

a) how many bits are defined for network number portion

b) how many bits are defined for host number portion.

(Be sure not to include any spaces in your answers. Just indicate the number for each answer.)

Answer 1

a)  17
b)  15

Explanation:
-------------
255.255.128.0
Let's convert all octets to binary separately
Converting 255 to binary
Divide 255 successively by 2 until the quotient is 0
   > 255/2 = 127, remainder is 1
   > 127/2 = 63, remainder is 1
   > 63/2 = 31, remainder is 1
   > 31/2 = 15, remainder is 1
   > 15/2 = 7, remainder is 1
   > 7/2 = 3, remainder is 1
   > 3/2 = 1, remainder is 1
   > 1/2 = 0, remainder is 1
Read remainders from the bottom to top as 11111111
So, 255 of decimal is 11111111 in binary
255 in binary is 11111111

Converting 128 to binary
Divide 128 successively by 2 until the quotient is 0
   > 128/2 = 64, remainder is 0
   > 64/2 = 32, remainder is 0
   > 32/2 = 16, remainder is 0
   > 16/2 = 8, remainder is 0
   > 8/2 = 4, remainder is 0
   > 4/2 = 2, remainder is 0
   > 2/2 = 1, remainder is 0
   > 1/2 = 0, remainder is 1
Read remainders from the bottom to top as 10000000
So, 128 of decimal is 10000000 in binary
128 in binary is 10000000

Converting 0 to binary
Divide 0 successively by 2 until the quotient is 0
Read remainders from the bottom to top as
So, 0 of decimal is 00000000 in binary

====================================================================================
||    255.255.128.0 in binary notation is 11111111.11111111.10000000.00000000    ||
====================================================================================

number of ones = 17
number of zeros = 15