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Same subnet, different subnet mask, direct communication?

Posted on 2006-10-23
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Last Modified: 2011-09-20
Hi Guys,
    I'm wondering if two computers on the same subnet can still communicate directly with each other, ie not through a router, if one has its subnet mask configured incorrectly, for example if the subnet of computer A is correctly 255.255.224.0 and the subnet of computer B is 255.255.0.0.
Yeh, i know that normally computers on different subnets have to go through a router or the default gateway but what if the subnet was configured like above?

Cheers,
phram3z
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Question by:phram3z
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Assisted Solution

by:Chris Dent
Chris Dent earned 64 total points
ID: 17787481

Only if they are within the same most restricted mask.

Any traffic outside the mask range will be sent to the Default Gateway (unless a static route has been specified), the route back just doesn't work.

Chris
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Steve Knight earned 192 total points
ID: 17787580
Agreed.  They would have to be within the 255.255.224.0 mask, e.g. if you were using
10.0.0.0 ranges then

Devices all within the range 10.0.0.1 to 10.0.31.254 can communicate
Likewise 10.0.32.1 to 10.0.63.254 etc.

In situations where mask or gateway has been set wrongly easiest remote fix is to jump onto a box within the same subnet and reconfigure it from there.

In your case as the mask has been set too big it will communicate fine within it's local LAN but another IP (such as 10.0.127.1 in the example above, e.g. a device on another subnet or over a WAN) won't be able to communicate with it because packets won't return to the default gateway as they will be seen as local.

If a WAN link goes to another address range outside the 255.255.0.0 mask, again not an issue.

Any particular problem here or just after a bit of background?

Steve
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Author Comment

by:phram3z
ID: 17787642
thanks doods =P quite helpful
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LVL 71

Expert Comment

by:Chris Dent
ID: 17787673
Pleasure :)

Chris
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LVL 43

Expert Comment

by:Steve Knight
ID: 17787766
No problem. Steve
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