Re: Odd & Even ACLs

Awesome, what a great binary lesson by Scott, it is just simply great...One
just hopes it doesn't get that complicated in the lab.

Ravi, great notes thanks a lot mate.. I still have one question though, the
starting bit (0 for even & 1 for odd), you say "Therefore, the network
address would become 1.1.0.0 ( which is an AND of the binary bits )" . Do
you mind elaborating further? Everything else makes sense.

On Tue, Jul 7, 2009 at 12:42 AM, Ravi Singh <way2ccie_at_googlemail.com> wrote:

> 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 Tue Jul 07 2009 - 01:09:49 ART