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Beginner

## Subnetting Quiz Prob

Q. You have an IP of 156.233.42.56 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

Explanation:

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 ?

3 REPLIES 3
Beginner

This is because there are two unusable subnets

all zero's and all one's

Hi,

`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.

Regards.

Alain.

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Beginner

Here are some resources to learn and practice subnetting:

http://subnettingquestions.com

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