Odd & Even ACLs
Hi Molomo,
Its just one of those shortcut steps to calculate the wildcard masks for ACLs ( courtesy : IE Blog again ) . Therefore , you might be interested in reading this
P.S – There is a difference in how I remember the output of a XOR operation than the one specified on the blog, so whichever way you wanna remember it ..Do go through the comments in the blog..there is a nice explanation by a gentleman called Paul ..
Regards, Ravi
On Tue, Jul 7, 2009 at 12:09 AM, Molomo wrote:
> 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 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 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 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 >>>> >>>> >>>> >>>> _______________________________________________________________________