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Subnetting_why are these two subnets talking?

Posted on 2007-11-19
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Last Modified: 2012-06-21
Hi.  I expanded my (single - unrouted) network over the weekend by changing the subnet from 255.255.255.0 to 255.255.240.0.  I am fairly new to networking but thought that these were two separate subnets and so would not easily talk to each other.  I'm glad they do as my job was that much easier but why, during the changeover, did devices on each subnet still communicate with each other?  Will I lose connectivity at some point if I missed a subnet change on one of my devices.
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Question by:valmatic
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MorDrakka earned 75 total points
ID: 20313059
Hi,

I think that is the case because the fact the two subnets overlap eachother if you have the same starting IPadress.

Let me explain:

Say at first you have this:

http://jodies.de/ipcalc?host=10.0.0.1&mask1=24&mask2=

And you are migrating to this:

http://jodies.de/ipcalc?host=10.0.0.1&mask1=20&mask2=

If you compare the 2 start hostname's with eachother you see they overlap eachother.

Does this clarify somewhat ?

M


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Author Comment

by:valmatic
ID: 20313307
I understand what you mean about the overlap and that looks like exactly what's happening but I'm still unclear as to why that occurs.  (?)  
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Assisted Solution

by:Johnjces
Johnjces earned 75 total points
ID: 20313478
That is the purpose of subnetting. You can easily allow several sub-networks to talk.

Get an IP calculator. That will show you easily the allowed ranges that can be created using a different IP subnet mask.

Here is a link for an online one:

http://jodies.de/ipcalc

Here is a free one from 3Com:

ftp://ftp.3com.com/pub/utilbin/win32/3CIPCalc.zip

Might help

John
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by:valmatic
ID: 20313508
THanks guys,  Guess that answers my question. :)
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Expert Comment

by:MorDrakka
ID: 20314151
WHAT ? I believe I answered it. Johnices even gives the same link as I do.
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by:Johnjces
ID: 20314196
MorDrakka

I am so sorry! I did not read your post or I would have not included the online IP Calculator.

The only difference was a direct answer to his question, "...but why, during the changeover, did devices on each subnet still communicate with each other? " with; "That is the purpose of subnetting. You can easily allow several sub-networks to talk."

John
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Expert Comment

by:Rob Williams
ID: 20314674
As an "innocent bystander", I would say MorDrakka deserves a point split as a minimum. If you like you can ask the moderators to re-evaluate in the community support forum.
http://www.experts-exchange.com/Community_Support/General/#browseZones
--Rob
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Expert Comment

by:Johnjces
ID: 20315046
valmatic

I do concur with RobWill!

John
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by:valmatic
ID: 20320252
points split.  
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