Question

On a Class C network, how many hosts per subnet will you get if you have a subnet mask of 255.255.255.240. On how many subnets was divided this class C address? Show your calculations

Answer 1

255.255.255.240
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
255 in binary is 11111111

Converting 240 to binary
Divide 240 successively by 2 until the quotient is 0
240/2 = 120, remainder is 0
120/2 = 60, remainder is 0
60/2 = 30, remainder is 0
30/2 = 15, remainder is 0
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 11110000
240 in binary is 11110000

so, 255.255.255.240 in binary notation is 11111111.11111111.11111111.11110000

first 28 bits of this subnet mask are 1's. remaining 4 bits are 0's.

1)  Number of hosts per subnet
=>  2^(number of 0 bits) - 2
=>  2^4 - 2
=>  16 - 2
=>  14
Answer: 14

2)  On how many subnets was divided this class C address
=>  number of subnets in class C address = 2^number of 1's in last octet (11110000)
=>  2^4
=>  16
Answer: 16