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# Tackling a subnetting question which I can't get my head round

### #1 Stryker06 Posted 18 November 2005 - 04:12 AM

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I've got a network address - a class c address, and I am suposed to divide into 7 subnets. Now from what I know, it's not possible to create 7 subnets? correct?
if so, would anyone be able to tell how it's done? or direct me to a tutorial if the explanation is deemed too long.
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### #2 brianmil0923 Posted 18 November 2005 - 07:59 AM

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Hey Stryker06,

What is the address? In order to solve this problem, you will need to use a custom/hard subnet mask. In other words it will not be one of the standard subnet masks, follow this LINK for an example.

dontcha just love binary math.......

Edited by brianmil0923, 18 November 2005 - 08:07 AM.

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### #3 dsenette Posted 18 November 2005 - 08:32 AM

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you can make any number of subnets you like (within reason...and within the bounds of the math involved)...doesn't matter if it's even numbers..odd numbers...
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### #4 Stryker06 Posted 18 November 2005 - 06:54 PM

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I'm doing a practice test for Network+ certification. The address is 197.72.143.0

I've discovered that you can use the 6 = 8-2 rule, and use the first and last subnet as well. So the first subnet would be from 197.72.143.0 - .31 , the second subnet from .32 - .63 and so on. And then have room for one more subnet at the end.
I was wondering though what the gateway address is supposed to be now that I'm using the first subnet. Usually, the gateway with an address like this would be 197.72.143.1, right?, well, that address now happens to be a node address. So what is the gateway fro each subnet going to be now?
Sorry if this is all too basic for some of you, but bear with me plz.
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### #5 brianmil0923 Posted 18 November 2005 - 10:45 PM

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The gateway address will be determined by whichever address from each subnet you assign to the ethernet interface on each core router. Keep in mind you cannot use the broadcast address in each subnet.

197.72.143.31
197.72.143.63
197.72.143.95
197.72.143.127
197.72.143.159
197.72.143.191
197.72.143.223
197.72.143.255

Normally, you would use the first assignable address from each subnet for this purpose. A subnet mask of 255.255.255.224 (or /27 for those of you who 'habla cisco ) will give you 8 subnets with 32 hosts per subnet.

In practice, you would then only use 7 subnets with one availible for expansion. As far as the math goes, you cannot have only 7 (has to be a power of 2)

TGIF (got out of work)....WOOT..sorry so short earlier//
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### #6 Stryker06 Posted 19 November 2005 - 06:59 AM

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Okay, so if I understand it correctly, can a host be used as a gateway as well? e.g, can address 197.72.143.1 be used as both?
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### #7 brianmil0923 Posted 19 November 2005 - 07:30 AM

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Yes a host address can be used. a host is just a computer,router,fax machine,copier,toaster...whatever ..that you assign an IP address to in an IP network.

Your gateway is a ROUTER, or more specifically, the IP address for the interface on the router to which a given subnet is connected. (which, by the way, is what you would set as the DEFAULT GATEWAY for that subnet)
In your case, 197.72.143.1 would be assigned to the Ethernet Interface on the router for subnet 1.

And for each of the computers connected to the network on subnet 1. 197.72.143.1 would be set as the default gateway in the tcp/ip settings.

Edited by brianmil0923, 19 November 2005 - 07:32 AM.

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