Aggregating multiple class C networks

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Aug 5th, 2008
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I know CIDR can be used to aggregate multiple classfull networks. Is the use of supernetting recommended to "build" a larger network by changing the subnetmask of multiple class C networks. For example I could change the subnet mask on hosts in class C networks 192.168.1.0, 192.168.2.0, etc. from 255.255.255.0 to 255.255.248.0. I have done this on few PC (WinXP) and an ethernet interface on a router (ip classless) and each can ping the other. Is this a recommended approach? Any issues that this might cause?

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4rmorris Tue, 08/05/2008 - 16:12
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Nothing prevents you from doing this. If you have a company standard that assigned 192.168.0.0/20 to a campus, and you wanted to use 192.168.0.0/23 for one floor in a building and 192.168.0.2/23 for another, CIDR allows you to do this (and to aggregate as well). I believe some older protocols (RIP?) don't support supernetting, but if you're using new software there's nothing wrong with this approach.


Good luck,


Ryan

Edison Ortiz Tue, 08/05/2008 - 16:14
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You are expanding your broadcast domain by going from /24 to /22.


The recommended approach is to break the IP subnets as smaller as possible.


1) Better IP Address preservation

2) Better overall traffic (less broadcast in the broadcast domain)

3) Better control in case of a virus or denial of service.


When you have a large broadcast domain, everyone connects within that domain in Layer2 and you don't have a device in Layer3 protecting intermediate devices. With today's L3 switches, you can perform inter-vlan routing as fast as pure L2 switching.


HTH,


__


Edison.

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