# # Useable Subnets ?

May 24th, 2007

When doing my exercise i was asked : how many useable subnets and hosts will the following network address allow?

The choices were between :

A. There 7 useable subnets with 30 useable host addresses

B. There 7 useable subnets with 2046 host addresses

C. There 7 useable subnets with 8190 useable host addresses

D. There 8 useable subnets with 30 useable host addresses

E. There 8 useable subnets with 2046 useable host addresses

F.There 8 useable subnets with 8190 useable host addresses

The answer the book's author gives is F but I answered C.

considering we have 13 host bits, that leads to 2^13 minus 2=8190 and the 3 subnet bits give 2^3 minus two reserved , thus 6 subnets. But we're allowed for using zero subnet, so one can have 7 subnets.

Can also we use the broadcast subnet number?

I will pass the CCNA exam next week, what will i answer if the question want me to tell how many subnets are useable, given n subnets bits? (2 power n minus 2 or not?)

Overall Rating: 0 (0 ratings)

## Replies

compsolv Thu, 05/24/2007 - 13:01

The formula that you should use for calculating subnets is simply 2^n. There is no longer a need to exclude the all zeros and all ones subnets.

Anthony J. Sequeira

CCIE #15626

CSCO10892433 Thu, 05/24/2007 - 19:05

Hi, farellfolly

172.31.0.0 is a class B network. With /29 mask, there will be 13 bits for subnet and 3 bits for host. So the usable subnets would be:

2^13=8192

and the usable hosts would be:

2^3-2=6

So the answer to this question is

G. none of the above. :-)

As for the formula to use, here is a strategy I would suggest to you:

1.If the question explicitly indicate that ip subnet-zero is used, use 2^n for the subnet and 2^n-2 for the host.

2.If the question explicitly indicate that ip subnet-zero cannot be used, use 2^n-2 for the subnet and 2^n-2 for the host.

3.If the question does not explicitly indicate whether subent-zero can be used or not, use both formula to calculate two vlaues( 2^n and an-2) for the subnet and 2^n-2 for the host. Then eliminate the option that has value other than these vlaues.if you are lucky, there would be only one option left to you to choose. Don't try to choose an answer which has value like 3,7,15 etc. They cannot be right.

4.If, unfortunately, there are still two option left, use the 2^n formula because in real life, there is no more limit on subnet-zero for newer device. It is a good chance for you to get the score from this question.

HTH

SSLIN