determining the subnet mask for a required number of subnets

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Hi guys, I'm having a bad time through this chapter in the 70-291 book where I need to determine the subnet mask for a required number of subnets, the book says I need to determine the subnet mask value that is b bits removed from and after the original subnet id for example 255.255.255.0 and b=3, your subnet mask is 255.255.255.224 because the ascending order of subnet mask values, that is the subnet mask that is 3 bits removed from and after 255.255.255.0.

I simply can't understand how can I remove 3 bits from 255.255.255.0 and end with 255.255.224.0, so far I know taking 3 bits will result in .248 subnet

This part of the book is really confusing, there is nothing explain very well what happens for me. Can someone help me please?

 

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Have a look at my posts here for (hopefully) a better idea of subnetting.

 

I think of it in this way:

Subnet mask No. of bits needed

128 1
192 2
224 3
240 4
248 5
252 6
254 7

If working out subnet questions I always write down this table as a quick guide. So you could reword your problem as "How many bits are left after taking away 3 bits?" Then refer to the table to get your subnet mask.

 

damn, I was studying until so late that I didn't notice I was confusing the 3rd and 4th octets thinking the 4th was always on the 3rd place, because of this the 255.0+3bits is 255.224... my great fault!

I'll get some more sleep, this subnet subject consumed all my brain this weekend.
thanks!

 

I think I see where the confusion lies.
The question states
the subnet mask value that is 3 bits removed from and after the original subnet id 255.255.255.0
Not
the subnet mask value that has 3 bits removed from the original subnet id 255.255.255.0
I read it the second way a few times. I guess that shows the importance of reading and understanding the question before you attempt an answer.
I still think it's a poorly worded question though, unless the intention is to test candidates on attention to detail as well as technical issues.

 

I am still trying to perfect subnetting so if I get this wrong please corect me.

I do a chart like this for an ip for example 192.168.1.165 and subnet mask of 255.255.255.0

11000000.10101000.00000001.10100101
11111111.11111111.11111111.00000000
---------------------------------------
11000000.10101000.00000001.00000000
where there is 1 on the top line and a 1 on the bottom line you put a 1 and if its two 0 or a 1 & a 0 you put 0

This makes your network address which is 192.168.1.0

To get the broadcast you change all the 0 in the last octet to 1 so the broadcast is 192.168.1.255

Now to work out the hosts you take 2 away 1 is reserved so just take 1 away from the 255 which makes 254 so the host address range is 192.168.1.1 - 192.168.1.254

So for a /26 subnet with an ip of 192.168.1.0

11000000.10101000.00000001.00000000
11111111.11111111.11111111.11000000
---------------------------------------
11000000.10101000.00000001.11000000

I wont type it all out by as you can see hopefully that you can have a max of 4 networks with max number of hosts being 63 but one is reserved. so its 62.

Hopefully that makes sense.

 

If ur actually having problems on how to subnet. Then check this (www.subnetting-secrets.com) out and it wont be a problem anymore.

 

It's easier just to remember that number of hosts=2^n-2, where n=number of host bits.
The minus two is the network and broadcast addresses.
I think this is a better method because you can easily work out 2^n if you know some of them.
Take as an example, a class B address with 20 network bits. That gives you 12 host bits. I don't know what 2^12 is, but I know what 2^10 is (1024) and can multiply by 2 twice (2048, 4096) really quickly and then subtract two. You can't do such simple maths with your method.