OK, I think I got it.
/15 doesn't make sense if I want to pick up just a 10.2.0.0 network because
it would also pick up a 10.3.0.0 network.
/16 will work if its the intent to summarize at 10.2.0.0, however that over
summarizes if its not our intent to pick up 10.2.0.0
Therefore the three addresses:
10.2.1.0/24
10.2.2.0/24
10.2.3.0/24
can only be summarized as:
10.2.1.0/24
10.2.2.0/23
Thanks to all for your responses, they helped loads (especially if I got it
right this time)
Steve
""Howard C. Berkowitz"" wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> >I want to summarize three addresses within an OSPF area:
> >
> >10.2.1.0/24
> >10.2.2.0/24
> >10.2.3.0/24
> >
> >Converting to binary, I see the 15th bit is the highest order bit the
three
> >addresses have in common. From that I see the summary address is
10.2.0.0.
> >What I don't understand is why the subnet mask is 16bits. To me it looks
> >like it should be 15.
> >
> >Thanks,
> >Steve
>
> There's a widespread and unfortunate belief that "summarization" is
> OK if it includes a list of addresses, even if it picks up addresses
> not included in the list. True, you can probably get away with that
> in many enterprise situations, but it can be catastrophic in the
> Internet.
>
> As I interpret the problem, the minimum number of addresses that can
> be created from your example are:
>
> 10.2.1.0/24
> 10.2.2.0/23
>
> You can't do the /22, because it would pick up 10.2.0.0/24, which
> isn't part of the list.
>
> /15 or /16 make no sense. Converting the third octet to binary:
>
>
> 10.2.0.0/24 000000 00
> 10.2.1.0/24 000000 01
> 10.2.2.0/24 000000 10
> 10.2.3.0/24 000000 11
>
> ^^^^^^ ^^
> 111222 22
> 789012 34
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