Re: Odd & Even ACLs

As I was reviewing my sent mail I realised a very small typo in the odd
subnet section .. the line should say ..Converting them to binary shows that
for the octet to be "ODD" the last bit in the octet is always 1.

On Mon, Jul 6, 2009 at 11:38 PM, Ravi Singh <way2ccie_at_googlemail.com> wrote:

> See if this helps ..Just a paste of some quick notes that I had prepared
>
> Filtering routes with an even subnet e.g for a subnet 1.1.0.0/16 , we need
> only routes with an even class C subnet for e.g 1.1.0.0/24, 1.1.2.0/24,
> 1.1.4.0/24 and so on . Converting them to binary shows that for the octet
> to be even the last bit in the octet is always 0
> 0 0 0 0 0 0 0 0 - 0
> 0 0 0 0 0 0 1 0 - 2
> 0 0 0 0 0 1 0 0 - 4
> 0 0 0 0 0 1 1 0 - 6 and so on
> Therefore, the network address would become 1.1.0.0 ( which is an AND of
> the binary bits ) and the wildcard mask will be 0.0.254.255 (which is an XOR
> of the binary values) In XOR an output is 1 only when all the bits are
> either 0's or 1's . Therefore , an XOR of the above values will be
> 0 0 0 0 0 0 0 1 . In another words this means that the values of the 7 bits
> can be anything but the 8th bit has to be an exact match. This when
> represented in a wildcard mask gives a value of 254.
>
>
> Filtering routes with an odd subnet e.g for a subnet 1.1.0.0/16 , we need
> only routes with an even class C subnet for e.g 1.1.1.0/24, 1.1.3.0/24,
> 1.1.5.0/24 and so on . Converting them to binary shows that for the octet
> to be even the last bit in the octet is always 1.
> 0 0 0 0 0 0 0 1 - 1
> 0 0 0 0 0 0 1 1 - 3
> 0 0 0 0 0 1 0 1 - 5
> 0 0 0 0 0 1 1 1 - 7 and so on
> therefore an XOR for the above values will be the same i.e
> 0 0 0 0 0 0 0 1, what changes though is the AND of these values which means
> the network address of the subnet. An AND of the binaries is
>
> 0 0 0 0 0 0 0 1 , which therefore will make use of the value 1.1.1.0 and
> the wildcard value of 0.0.254.255 .
>
> Regards,
> Ravi
>
>
>
>
> On Mon, Jul 6, 2009 at 8:39 PM, Molomo <letjedilakopa_at_gmail.com> wrote:
>
>> Experts,
>>
>> I'm really battling to understand how the network and the wildcard bits
>> are
>> arrived at for odd and even octes on ACL.
>> Anyone cares to give brief explanation?
>>
>> Thanks in adavnce.
>>
>>
>> Molomo
>>
>>
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Received on Mon Jul 06 2009 - 23:42:15 ART