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subnetting problem

By cpfeiffe ·
I have a Unix system that is routing. It is a firewall. I am responsible for the OS. It has the following subnets/routes
10.10.0.0/19 > nextrouter
10.10.32.0/20 > nextrouter
10.10.48.0/21 > nextrouter
10.10.56.0/21 > interface (NIC)
10.10.64.0/19 > nextrouter
10.10.96.0/19 > nextrouter
This works fine. My question is why can't I do a 10.10.64.0/18 instead of using the two networks at the end of the list. I tried it with the /18 and it didn't work. Our networking manager quickly pointed out that I need two /19 masks because I have to be as specific or more specific than the 10.10.0.0 subnet. I can't find any documentation to support this and he wasn't really able to explain it clearly. Can anyone explain this better or point to a doc that does explain this and the other non-basic subnetting rules.

On another system I tried
10.100.0.0/18 > nextrouter
10.100.64.0/19 > nextrouter
10.100.96.0/20 > nextrouter
10.100.112.0/22 > nextrouter
10.100.116.0/24 > nextrouter
10.100.117.0/24 > interface
10.100.118.0/23 > nextrouter
This didn't work and I had to make the 10.100.118/23 network to /24 networks (118 and 119). Again, I wasn't able to get a good explanation.

I have to believe the guy knows what he's talking about because my method didn't work and his did. He spotted the fixes right away. To make things worse none of the network engineers (which he manages) believe him either. I'd like to understand for myself and also find some written support.

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subnetting problem

by Joseph Moore In reply to subnetting problem

First off, the best subnetting site is at:
http://www.learntosubnet.com

Go there to find out specifically why. But I agree with your manager. A more specific route will take presidence over a less specific route. So, since the 10.10.0.0/19 was already in place, the 10.10.64.0/18 is less specific and would not be used. It all comes down to the subnet bits for determining this.
I know I am not explaining this any better than your manager did, and I apologize. But that is just how subnettingworks! The website I listed will help you out greatly.

hope this helps

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subnetting problem

by cpfeiffe In reply to subnetting problem

Thanks. I looked at the video and it was pretty good, but still didn't validate what my nw manager was saying for the second example. Our router guys also point out that the 10.10.64 rule shouldn't apply because the 10.10.0.0/19 route only extends to 10.10.63.255 so since it doesn't cover 10.10.64 as part of its network there should be no problem with being more/less specific. Anyway, I've looked at over 100 onlien docs and can't find this anywhere so I'm starting to believe there is a problem somewhere else (i.e. the next router) that I don't know about. Everyone I've spoken to at Sun, SANS, Cisco say this should work.

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subnetting problem

by cpfeiffe In reply to subnetting problem

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