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NETWORK MASK USING 255.255.255.248

Posted on 2010-09-09
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Last Modified: 2012-05-10
We have a customer who has their T-1 provided by Paetec.   Paetec also provides our T-1 service.  As it turns out, our customer is on the same subnet as we are.  Both of us are on 63.252.84.xxx and problem is they cannot reach our webserver.   Paetec suggested we change our network mask from 255.255.255.0 to 255.255.255.248 to fix the issue.

I would love a clear understanding of why this will make it all workable?   Paetec could not or would not explain, just said do it.
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Question by:Lance McGrew
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Todd Gerbert earned 500 total points
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To over-simplify a bit ... the network mask allows for the division of an IP network into several smaller networks.
The subnet mask you should be using is given to you by the network administrator, in this case you're talking about the T1 network (which really is it's own network with two devices, your router and Paetec's router), and since Paetec is the administrator of that network you should be using the netmask they've given you - it's not something you get to "just choose" - it has to be what you're told to use.
Really, you are not on the same subnet as your customer - they are on the 63.252.84.0 - 63.252.84.3 network, and you're on the 63.252.84.4 - 63.252.84.7 network (each network has only 4 IP addresses), but since you're using the wrong netmask your router wrongly thinks 63.252.84.0 - 63.252.84.3 is on the same network, so it doesn't route packets the way it should.
Hope that makes some sense - your question is fairly broad, and routing/IP addressing can be somewhat involved.
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by:kf4zmt
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The subnet mask is what separates the host part of an ip address from the network part.  It looks like in your case your isp has split a class b network into multiple subnets to assign out to their customers.  When both you and your customer use the correct subnet mask, you won't actually be on the same subnet. Unless of course your isp has made a mistake but this is unlikely.
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