Q. You have an IP of 18.104.22.168 with a subnet mask of 7 bits. How many hosts and subnets are possible?
A. 126 hosts and 510 subnets
B. 128 subnets and 512 hosts
C. 510 hosts and 128 subnets
D. 512 subnets and 128 hosts
Correct answer: C
Class B network has the form N.N.H.H, the default subnet mask is 16 bits long.
There is additional 7 bits to the default subnet mask. The total number of bits in subnet are 16+7 = 23.
This leaves us with 32-23 =9 bits for assigning to hosts.
7 bits of subnet mask corresponds to (2^7-2)=128-2 = 126 subnets.
9 bits belonging to host addresses correspond to (2^9-2)=512-2 = 510 hosts.
But i what studied in Cbts Trainings the Formula to know the subnets is " 2 raise power n "
Where n is the number of subnets bits, so the answer subnets should be 128.
But in the explanation , it use formula to find the hosts ?
Kindly reply ASAP
This is because there are two unusable subnets
On Cisco gears these have been usable for a long time now with the ip subnet zero command which is default.