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# route summerization

Posted on 2004-09-02
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Hi

can some one tell me a quick and easy way of working out what subnets are included in a summerized route. E.g 172.17.200.0 /12. I don't want the answer to this example only I want to know how you did it and how can I work out this information if I was given other summerized addresses of any class with any bit network mask. Thank you in advance
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Question by:kamal73
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Expert Comment

ID: 11968946
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Expert Comment

ID: 11970201
the quick way is to write out the ip in binary. thne the last digit that matches in all the IP's is the point it summarizes on.
10.10.10.128     x.x.x.10000000
10.10.10.224     x.x.x.11100000
10.10.10.235     x.x.x.11101011

the routes would summarizes on 10.10.10.128 since 128 is the last digit that is commom to all.
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Accepted Solution

ID: 11970756
The best way to work out what networks are included in a sumarised route is to look a the mask in binary, in your example /12 means that the first 12 bits of the mask are 1s and the rest are 0s so all addresses with the same first 12bits as your summarised one will be included. i.e.

Hope that helps

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Expert Comment

ID: 11970762
I is really easy if the mask falls on an octet boundary, (8,16,24) it is the entire octet after the end of the mask e.g. for 16 bit mask it will be x.y.0.0 to x.y.255.255
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Author Comment

ID: 11971056

you lot are all probrbly right in your own way but I'm afraid I'm really thick when it comes to subnetting so please consider this when you are explaining, but once I get the concept I'm sure every thing will be cleared

I have a question which says

Which subnet address is included in the route summerization 172.17.200.0 /12

the choices I have are
172.17.98.0
172.17.206.0
172.17.106.0
172.17.224.0
172.17.235.0

what would be the answer and why ?
Sorry I am approaching this way, but I think my  brain would understands better in this way.
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Expert Comment

ID: 11971365
Is that really the question or have you picked an address at random?
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Expert Comment

ID: 11971517
I think you may have a bad question that's causing your confusion.

My guess would be that the person writing the question meant to say 172.17.200.0/21. Looks like they transposed the digits in the mask.

Try it with a 21 bit mask and see if you can come up with the answer.

-Don
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Expert Comment

ID: 11974216
All of them.

172.17.200.0/12 is invalid.  Anything from 172.16.0.0 thru 172.31.255.255 is included in 172.16/12.

--Tim
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LVL 28

Expert Comment

ID: 11974375
Oh Tim, you are SOOO not right. Just kidding :-) Welcome to EE!
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Author Comment

ID: 11979572
well it is an official exam question, and you are right it has confused me badly.
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LVL 50

Expert Comment

ID: 11979734
Unfortunately, even "official" questions have errors. :-(

-Don
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Author Comment

ID: 11981016
I'm still going to reward fatald  cause his methods seems the easiest.
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