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route summerization

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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kamal73
Asked:
kamal73
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1 Solution
 
MarkDozierCommented:
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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fatladCommented:
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.

Mask      11111111.11110000.00000000.00000000
Address  10101100.10010000.0.0 starting point (172.17.200.0)
Address  10101100.10011111.11111111.11111111 finsishing point (172.159.255.255)

Hope that helps

FatLad
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fatladCommented:
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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kamal73Author Commented:

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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fatladCommented:
Is that really the question or have you picked an address at random?
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Don JohnstonCommented:
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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tevensCommented:
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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mikebernhardtCommented:
Oh Tim, you are SOOO not right. Just kidding :-) Welcome to EE!
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kamal73Author Commented:
well it is an official exam question, and you are right it has confused me badly.
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Don JohnstonCommented:
Unfortunately, even "official" questions have errors. :-(

-Don
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kamal73Author Commented:
I'm still going to reward fatald  cause his methods seems the easiest.
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