What's wrong with my summarization procedure?

[news2010a]

Hi,

I am trying to advertise this into BGP. I will try to summarize this and if possible advertise it via one network statement.

In order to visualize the summarized network, I converted the third octet of networks above to binary:

9.176.210.0 255.255.255.0
9.176.211.0 255.255.255.0
9.176.212.0 255.255.255.0
9.176.213.0 255.255.255.0
9.176.214.0 255.255.255.0
9.176.215.0 255.255.255.128
9.176.216.0 255.255.252.0
9.176.217.0 255.255.255.0

So converting the third octet to binary:

11010010 = 210
11010011 = 211
11010100 = 212
11010101 = 213
11010110 = 214
11010111 = 215
11011000 = 216
11011001 = 217

Then I see that all these entries have up to the 4th bit in common (from left to right). Then if I follow that rationale, the summary route should be:

9.176.208.0/20

However, that is wrong because as you can see 9.176.208.0 and 9.176.209.0 should not be included on this.

What am I missing? What is the right approach to summarize such networks then? My methodology always worked before so I don't get what is wrong.

[Jon Marshall · Accepted solution]

news2010a wrote:
9.176.210.0 255.255.255.0 
9.176.211.0 255.255.255.0 
9.176.212.0 255.255.255.0 
9.176.213.0 255.255.255.0 
9.176.214.0 255.255.255.0 
9.176.215.0 255.255.255.128 
9.176.216.0 255.255.252.0 
9.176.217.0 255.255.255.0
So converting the third octet to binary:
11010010 = 210
11010011 = 211
11010100 = 212
11010101 = 213
11010110 = 214
11010111 = 215
11011000 = 216
11011001 = 217
Then I see that all these entries have up to the 4th bit in common (from left to right). Then if I follow that rationale, the summary route should be:
9.176.208.0/20

However, that is wrong because as you can see 9.176.208.0 and 9.176.209.0 should not be included on this.
What am I missing? What is the right approach to summarize such networks then? My methodology always worked before so I don't get what is wrong.

Marlon

Some network ranges are not summarisable in one statement and the above is true for you.

Also note that 9.176.208.0/20 covers

9.176.208.0  ->  9.176.23.255  so it also includes a whole lot more at the other end. You can't summarise the above with one statement without including a lot more /24s.

Jon