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# Route summarization

Posted on 2016-10-31
Medium Priority
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Which summary route can  summarize the following network
172.16.10.0/24
172.16.20.0/24
172.16.30/024

Can I summarize it as 172.16.10.0 / 19 ?

Thx
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Question by:AXISHK
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LVL 31

Expert Comment

ID: 41867913
Network address for summary is 172.16.0.0 in this case, subnet mask is good.

172.16.0.0/19 <-- address range  172.16.0.0 - 172.16.31.255
So all networks are included, however, there are many unused network ranges included.

Consider this for example:
If you want summarize 192.168.1.0/24 and 192.168.2.0/24 summarization will be done in this fashion:
192.168.0.0/24
192.168.1.0/24
will be summarized with 192.168.0.0/23
192.168.2.0/24
192.168.3.0/24
will be summarized with 192.168.2.0/23

Whole range will be covered with 192.168.0.0/22 (192.168.0.0/24 - 192.168.4.0/24). You don't start with smallest network address that is included in summarization, but with network address of whole range.

Networks 192.168.0.0/24 and 192.168.1.0/24 are summarized in 192.168.0.0/23 - networks that are not present will not be advertised. That's why planning of network address space is important. You need to start to think about it when you start to design your network to be able to have good summarizations.
Second part of this would be: divide network space per region from the start so you can have good summary address for each region.
Also when preparing network addresses for summary - you do not to start network addresses with power of 10 like you did 172.16.10.0, 172.16.20.0, 172.16.30.0, but choose network addresses as power of 2 (0, 1, 2, 4, 8, 16, 32, 64 etc...).
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Author Comment

ID: 41867923
But giving that 172.16.10.0/24,  172.16.20.0/24 and 172.16.30/024 need to be summarized in the question. Should the only best solution is 172.16.10.0 / 19 which covers IP range from 172.16.0.1 - 172.16.31.254, correct ?

Thx again.
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LVL 31

Accepted Solution

Predrag earned 2000 total points
ID: 41867925
Not correct. 172.16.0.0/19 is summary. That one is covering IP address range 172.16.0.1 - 172.16.31.254.

I am trying to give you wider picture in explanation above. :)
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Author Comment

ID: 41867935
Here is my calculation, please comment.

1. Convert the 3rd octet in the networks 172.16.10.0, 172.16.20.0, 172.16.30.0  to binary format.
10 : 00001010
20 : 00010100
30 : 00011110
2. The common pattern of digits in 3rd octet ends is "000". Hence, the first 19 bits of the IP address from 172.16.0.0 to 172.16.31.255 are the same. The best summary route is 172.16.0.0 /19 which will cover 172.16.10.0/24,  172.16.20.0/24 and 172.16.30/024 although it will also include network that are not presented.
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LVL 31

Expert Comment

ID: 41867942
That's it.
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