Re: Odd & Even ACLs

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 - 23:38:48 ART