Summarization doubt...

besh6274

Hi all ladies and gentlemens...

i'm in the middle refreshing my route summarization knwoledge.

i found something is funny...

can someone explain to me. WHY the following subnet cannot be summarize into 1 summary?

_______This is what i got from cisco website - for discussion purpose only_______
Consider this set of networks for further explanation.

192.168.146.0/24
192.168.147.0/24
192.168.148.0/24
192.168.149.0/24

The first two octets and the last octet are the same for each network. This table is an explanation of how to summarize these.

The third octet for the previous networks can be written as seen in this table, according to the octet bit position and address value for each bit.

What does those M M M M M ? ? ? meants? and M M M M M D D D

Why can't we summary it into 192.168.144.0/21 ? 192.168.144.0 255.255.248.0

Unlike previous example, you cannot summarize these networks into a single network. You need a minimum of two networks. The previous networks can be summarized into these two networks:

For networks 192.168.146.x and 192.168.147.x, all bits match except for the last one, which is a "don't care". This can be written as 192.168.146.0/23 (or 192.168.146.0 255.255.254.0).

For networks 192.168.148.x and 192.168.149.x, all bits match except for the last one, which is a "don't care". This can be written as 192.168.148.0/23 (or 192.168.148.0 255.255.254.0).

dtlokee

You can create a single summary of 192.168.144.0/21 but that would include all of the networks from 192.168.144.0/24 to 192.168.100.151/24. The example only included the networks of 192.168.146.0/24 - 192.168.149.0/24 so your solution overlaps extra networks. Creating the larger summary may not be a problem in an enterprise network where you have control over the network and can rely on longest match to choose the individual routes to destinations that are overlapped by the summary. If you are advertising out to the Internet, the customers who have those addresses you are overlapping are not going to be very happy with you when you hijack their traffic (assuming your ISP doesn't properly filter it out, which is a good assumption).

besh6274

Hi dtlokee;

thanks for your explanation.

but i'm not so clear about that. would you mind to show me your calculation and also the ideal ans for this ? and how u manage to find the ideal ans ?

thanks alot.

dtlokee

I apply the same logic I would if I were subnetting using incremental values (eg a /21 would have an incremental value of 8, a /22 will have an incremental value of 4 and so on)

So to summarize the addresses:

192.168.146.0/24
192.168.147.0/24
192.168.148.0/24
192.168.149.0/24

Let's look at the multiples of 8 around 146 (/21)

144, 152, 160...

Multiples of 4: (/22)

144, 148, 152, 156...

Multiples of 2: (/23)

144, 146, 148, 150, 152, 154..

So looking at that we can see the only way to fit the addresses into summary ranges will be to use 2, 192.168.146.0/23 and 192.168.148.0/23. There is no way to fit the addresses into a single range without overlapping other ranges.

besh6274

thanks alot