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How Many Subnets?


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#1 jp2007

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Posted 11 September 2007 - 12:32 AM

Hi everyone. New to these forums and would just ike to say thanks for all the available resources here, they have helped a lot.

Was wondering whether someone could clear something up for me. Basically I'd like to know how you calculate how many subnets you can have. I've been reading round a few places and getting a lot of conflicting information that is confusing me.

Say you wanted six subnets. Is the calculation 2^3 -2 = 6 correct? Some places I see the calculation as having to minus 2 subnets and others where you dont minus 2 so end up with eight subnets. 2^3 = 8. On a previous post here I saw it was 2^x -1.

I know you use the 2^x -2 when calculating how many hosts you can have in each subnet because take two off for the network and broadcast addresses, but if someone could tell me what calcuation is actually used for the number of subnets I'd greatly appreciate it.

Edited by jp2007, 11 September 2007 - 12:34 AM.

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#2 Alice

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Posted 11 September 2007 - 01:26 AM

check out this explanation on this thread for subnetting by quinnessential!  smile.gif

http://www.proprofs.com/forums/index.php?s...is+on+my+subnet
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#3 jp2007

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Posted 11 September 2007 - 01:55 AM

Thanks alice. I have read that post and it shows a great way of calculating the number of networks and hosts on the networks although the second example is a bit confusing.

This is a quote from the example.

"Because your subnet range is 255.255.240.0 and it is a class b subnet it means you can have a total of 16 networks that can consist of 16 host addresses and out of those 16 host addresses 2 are reserved for the broadcast address (the last number in the range) and for the routers address (the first number in the range).

We work this out by doing the following 256 - 240 = 16 which gives you the amount of addresses each network will contain. Then you take 256 / 16 = 16 which gives you the amount of networks your subnet will have."

I agree with the amount of networks you will have which is 16, but I thought the amount of addresses each network will contain is 4094? 16x256-2 = 4094.

Lol I'm confused!!!


Edited by jp2007, 11 September 2007 - 02:24 AM.

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#4 Alice

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Posted 11 September 2007 - 02:24 AM

There are also tutorials on subnetting if you do a subnetting search you will find them. Don't make it to hard for yourself  smile.gif. When I was learning subnetting I had a hard time and then I saw quinns example and it seemed so easy!

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#5 Quinntessential

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Posted 11 September 2007 - 05:10 AM

QUOTE(jp2007 @ Sep 11 2007, 11:55 AM) <{POST_SNAPBACK}>
Thanks alice. I have read that post and it shows a great way of calculating the number of networks and hosts on the networks although the second example is a bit confusing.

This is a quote from the example.

"Because your subnet range is 255.255.240.0 and it is a class b subnet it means you can have a total of 16 networks that can consist of 4096 host addresses and out of those 4096 host addresses 2 are reserved for the broadcast address (the last number in the range) and for the routers address (the first number in the range).

We work this out by doing the following 256 x 240 = 4096 which gives you the amount of addresses each network will contain. Then you take 256 / 16 = 16 which gives you the amount of networks your subnet will have."

I agree with the amount of networks you will have which is 16, but I thought the amount of addresses each network will contain is 4094? 16x256-2 = 4094.

Lol I'm confused!!!

jp2007

Thanks for pointing that out indeed you are correct that the the number of host addresses would be 4096 with two reserved for the broadcast address and the router address.  

I have corrected it now I apologize for any inconvenience caused. blush.gif

Edited by Quinntessential, 11 September 2007 - 06:25 AM.

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#6 jp2007

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Posted 11 September 2007 - 05:54 AM

No problems  smile.gif . I think there may be something else that needs correcting.

"We work this out by doing the following 256 - 240 = 16 which gives you the amount of addresses each network will contain. Then you take 256 / 16 = 16 which gives you the amount of networks your subnet will have."

I think the first part of that calculation should read 256 * 16 -2 = 4094.

Anyways I've gone back to binary calculations like the example below as it seems to work better for me!  smile.gif

255.255.240.0
11111111.11111111.11110000.00000000

2^4 = 16 subnets
2^12 -2 = 4094 hosts

Still not entirely sure whether you have to use 2^x-2 for effective subnets so 2^4 -2 = 14 subnets, as some people seem to use it and some don't. From what I've read it seems it was an older method for subnetting but as technology has progressed the -2 has been dropped from the formula.

Heres an interesting article on it.

http://www.mcmcse.com/articles/subnetting.shtml

Also another article outlining the problem of not knowing what methods certification vendors use. Seems to be an old article but still relevant.

http://www.mcmcse.com/articles/wronginfo.shtml

Edited by jp2007, 11 September 2007 - 06:21 AM.

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