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IP address summary

Posted on 2015-01-12
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Last Modified: 2015-01-21
I am trying to figure out why the 2 prefixes below would summarize 192.168.1.0/24, .2.0/24, and .3.0/24:

192.168.0.0/21 and 192.168.0.0/22
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Question by:leblanc
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David Johnson, CD, MVP earned 167 total points
ID: 40546024
192.168.0.0/21 doesn't 192.168.0.1-192.168.7.254 broadcast 192.168.7.255
192.168.0.0/22 does      192.168.0.1-192.168.3.254 broadcast 192.168.3.255
You need to understand how CIDR works
a prefix size of /22 gives: 2^32-22 = 2**10 addresses 1024
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by:Don Johnston
Don Johnston earned 333 total points
ID: 40546472
The best you could accomplish is

192.168.1.0/24
192.168.2.0/23 (which includes 2.0 & 3.0)

There is no way to summarize all three networks with a single prefix (without including networks that shouldn't be included).
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by:leblanc
ID: 40560871
"192.168.0.0/21 doesn't 192.168.0.1-192.168.7.254 broadcast 192.168.7.255". I am not sure I understand this. It looks like /21 cover .1.0, .2.0, and .3.0/24
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by:Don Johnston
Don Johnston earned 333 total points
ID: 40560950
The reason why 192.168.0.0/21 won't work is because you are over summarizing.  That prefix includes networks:

192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
192.168.5.0/24
192.168.6.0/24
192.168.7.0/24

So while the prefix does include 1.0, 2.0 and 3.0, it also includes a bunch of networks that you don't want advertised.
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by:leblanc
ID: 40560999
ah... Got it.
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