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Route summaries with un-natural subnet boundaries

I have a customer who owns a class-b address. Over the years they've used a 255.255.252.0 mask for every subnet and they are all over the class-b range. Now we want to develop a distribution layer and break everything down into 9 distribution routers. I want to set up each distribution switch/router to have a 255.255.240.0 or /20 range of contiguous addresses which we'll break down into several /24's, /25's, /26's etc.

I would like to start the /20 ranges on natural boundary’s like x.x.0.0, x.x.16.0, x.x.32.0 etc., but the way they used random subnets, I can't get enough for 9 distribution switches. Can I start a /20 range at x.x.17.0 and then summarize routes to this range without causing havoc in the router tables. I'm thinking the router really doesn't care if it’s a natural boundary or not. Its just looking at the bits in the mask and doing the math for the range of addresses. As long as nothing overlaps existing space, I should be OK, right? I'm just looking for a sanity check...Thanks!

1 REPLY
Cisco Employee

Re: Route summaries with un-natural subnet boundaries

x.x.17.0/20 will be interpreted as x.x.16.0/20

xxxx xxxx.xxxx xxxx.0001 0001.0000 0000 prefix

1111 1111 1111 1111 1111 0000 0000 0000 mask

So if you intend x.x.16.0/20 someplace else, that could certainly be a problem.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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