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Router47
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Why is 172.16.50.0 the summarised address?

Mon Dec 02, 2013 4:40 pm

i have 8 network addresses:


172.16.50.0

172.16.60.0

172.16.70.0

172.16.80.0

172.16.90.0

172.16.100.0

172.16.110.0

172.16.120.0



my study material says 172.16.50.0 is the summarised address with 255.255.128.0 as the subnetmask


i understand why subnetmask is what it is, because only the first 17 bits in each network address match but i'm still hung up on why the summarised address itself is 172.16.50.0

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srg
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Re: Why is 172.16.50.0 the summarised address?

Mon Dec 02, 2013 5:06 pm

As you've more or less figured out, since this is a /17, 172.16.50.0 isnt the summarized network. Its contained within the summarization though.
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Router47
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Re: Why is 172.16.50.0 the summarised address?

Mon Dec 02, 2013 5:22 pm

i'm still confused why 172.16.50.0 is going to be advertised though and not any of the others


is it just a a rule that the first network address is always the advertised address?

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Re: Why is 172.16.50.0 the summarised address?

Mon Dec 02, 2013 5:26 pm

Advertised in what? You're leaving quite a lot of context out :)

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Router47
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Re: Why is 172.16.50.0 the summarised address?

Mon Dec 02, 2013 5:41 pm

srg wrote:Advertised in what? You're leaving quite a lot of context out :)



in case there was a Router that needed to advertise a "summarized address" to a Main Router


here's the actual quote from my study material


"What summarised address would you advertise? I make it 172.16.50.0 255.255.128.0, or /17."

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Re: Why is 172.16.50.0 the summarised address?

Tue Dec 03, 2013 9:10 am

The correct summary address to advertise as a single network would be 172.16.0.0/17, now tell us why it would be so....
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Re: Why is 172.16.50.0 the summarised address?

Tue Dec 03, 2013 9:29 am

IPv6?

Router47
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Re: Why is 172.16.50.0 the summarised address?

Tue Dec 03, 2013 5:14 pm

ristau5741 wrote:The correct summary address to advertise as a single network would be 172.16.0.0/17, now tell us why it would be so....



Do you feel the guy from my study material made a mistake?


I'm thinking since all of those addresses have only the first 17 bits in common, a '0' in the 128 spot, that seems like it would make it what you said, 172.16.0.0

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Re: Why is 172.16.50.0 the summarised address?

Tue Dec 03, 2013 7:19 pm

It's 172.16.0.0 for sure. Only thing I can think of is if you summarize at 172.16.50.0 then your router would discard any packets that were less than the summary route? So a packet in the 172.16.40.0 network would be dropped(guessing here). You can try it and see what happens on a router. But the going by the rules, 172.16.0.0 is right.

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Steven King
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Re: Why is 172.16.50.0 the summarised address?

Tue Dec 03, 2013 7:37 pm

srg wrote:As you've more or less figured out, since this is a /17, 172.16.50.0 isnt the summarized network. Its contained within the summarization though.


This. 172.16.50.0/17 isn't a valid subnet ID.

172.16.0.0/17 is, and srg mentioned 172.16.50.0 falls into it.

If you want to be lazy, just pop it into a subnet calc to verify it's not correct:

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Re: Why is 172.16.50.0 the summarised address?

Fri Dec 06, 2013 9:21 am

mellowd wrote:IPv6?
There's your answer - you asked an IPv4 question in an IPv6 forum so whatever is posted in response to your question is automatically converted to IPv6 (IPv6 has lots of features like that) so doesn't make any sense in your IPv4 context.
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Re: Why is 172.16.50.0 the summarised address?

Fri Dec 06, 2013 12:02 pm

mellowd wrote:IPv6?


I didn't get this until I realized what thread we were in :P I thought you were screwing around.
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