Subnetting question

Bolton07

You have an IP of 156.233.42.56 with a subnet mask of 7 bits. How many hosts and subnets are possible?


This question was on a practice test i did.


The answer was 510 hosts and 126 subnets which I think looks wrong. Am I right? What is the correct answer?


Also what does a subnet mask of 7 bits mean in this case? I thought a class B address had to have 16 bits or more turned on.

This was the proper wording of the question by the way.


Thanks

Adam

willhi1979

Based on the answer and math, it looks like they are talking about a 23 bit subnet value which would be a Class B subnet and 7 bits borrowed for the subnet, 16+7. That would end up with 128 possible subnets and 512 possible hosts then you would subtrace 2 off the host value for the multicast and subnet address and end up with 128 subnets and 510 hosts.

jmritenour

The number of hosts is right - the number of subnets is 128, though.

What was the exact wording of the question? If it said the subnet mask is 7, then that's a poor way of putting it. The number of subnet bits being 7 would mean 7 extra bits are added on the the base subnet mask for a class B address of 255.255.0.0, which would be 255.255.254.0.

chmorin

Bolton07 wrote: »

You have an IP of 156.233.42.56 with a subnet mask of 7 bits. How many hosts and subnets are possible?

This question was on a practice test i did.

The answer was 510 hosts and 126 subnets which I think looks wrong. Am I right? What is the correct answer?

Also what does a subnet mask of 7 bits mean in this case? I thought a class B address had to have 16 bits or more turned on.

Thanks

Adam

Erm, please don't use big font...

Anyway, To answer your question... The question, in my opinion, does not give you correct information.

156.233.42.56/7, which seems to be what is implied by the question, give you 33 some odd million hosts. Which you could break up into as many subnets as you wanted.

We could make this really complicated and try and figure out where they got the answer. 510 hosts in 126 subnets... That is some 4 and change(4.04761904761904 hosts per subnet. No binary math in subneting would get you that number.

Lets say it was 4 hosts per subnet... which is 504 host address, most of which are not usable, and 126 subnets.

Alright, I am absolutely not going to vlsm 126 subnets unless I'm getting paid to. My point is, if they wanted an answer from you they should of worded the question differently.

For example:

You are given an IP Address of 156.233.42.56 and a subnet mask of 7 bits. How many hosts are available in your given subnet?

Or

You are given a network address of 156.233.42.0 and need to have 128 networks with 126 hosts per subnet. What would your subnet mask be?

Hopefully someone can back me up on this. but I'm with yah, makes no sense.

That would end up with 128 possible subnets and 512 possible hosts then you would subtrace 2 off the host value for the multicast and subnet address and end up with 128 subnets and 510 hosts.

The question wants 126 subnets, not 128.

jmritenour

You'd only get 126 subnets if you're dealing with some ancient equipment that can't handle all zero or all one subnets...

chmorin

jmritenour wrote: »

You'd only get 126 subnets if you're dealing with some ancient equipment that can't handle all zero or all one subnets...

Any cisco exam today will let you use every subnet(unless otherwise said), since this is a cisco forum I was assuming that.

mikej412

jmritenour wrote: »

You'd only get 126 subnets if you're dealing with some ancient equipment that can't handle all zero or all one subnets...

Or if they're looking at old questions back from the day before the fact that subnet zero became the default in IOS and was finally reflected in the training materials, practice questions, and the exam.

willhi1979

chmorin wrote: »

The question wants 126 subnets, not 128.

I know, I was thinking the answer was wrong, and that it would be 128 and not 126.