IP subnet

Unanswered Question
Jun 14th, 2010

I knw this is quite simpe, trickish too, what is the best way to summarise

192.2.1.0

192.2.3.0

192.2.5.0

I have this problem too.
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Federico Coto F... Mon, 06/14/2010 - 11:29

Hi,

You can summarize as follows:

192.2.0.0/21

However, the above includes other subnets as 192.2.0.x, 192.2.6.x, 192.2.7.x

So, if you see overlapping problems, better not summarize.

If the above won't cause problems, go for it.

Federico.

Giuseppe Larosa Mon, 06/14/2010 - 11:34

Hello Just,

you need to look at the third byte write it in binary

1 = 0000 0|001

3 = 0000 0|011

5 = 0000 0|101

you need to look at the common bits from the LEFT, that are the first 5 bits before '|' this means the inclusive aggregate (that includes also other subnets) is:

moving three bits to the left from /24 boundary  : 24-3 = 21 this is the prefix length

192.2.0.0/21

if you look at summary routes that do not include other subnets the answer is negative: there is no way to aggregate these subnets without including other ones because they are not contigous

the fact that all of them have the last bit set to 1 in common is not useful

Hope to help

Giuseppe

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