From: Vladimir Sousa (vladrac@gmail.com)
Date: Mon Nov 19 2007 - 21:49:00 ART
That's a really nice set of example thx.
Vlad
On 11/17/07, mgreenlee@ipexpert.com <mgreenlee@ipexpert.com> wrote:
>
> When matching ranges, you need to look at block sizes that are binary
> multiples.
> (256, 128, 64, 32, 16, etc.)
>
> Blocks must begin on a multiple of the block size.
> (for a block of 64, start at 0, 64, 128, or 192 for example. Starting at
> 64
> would be the range 64-127, for example)
>
> To match the entire range, all bits are 1 for "don't care"
>
> To match even/odd, you "care" about the last bit, so the last bit is zero.
>
> Block size - Mask to match all - Mask to match even/odd.
> 256 255 254
> 128 127 126
> 64 63 62
> 32 31 30
> 16 15 14
> 8 7 6
>
> Whether you are matching even or odd depends on what the regular part
> (non-mask) is matching
> 1 with a mask of 254 will match odds for the range 0-255
> 0 with a mask of 254 will match evens for the range 0-255
>
> A few additional examples
> 193 with a mask of 30 will match odds for the range 192-224
> 128 with a mask of 14 will match evens for the range 128-143
>
> Marvin Greenlee, CCIE #12237 (R&S, SP, Sec)
> Senior Technical Instructor - IPexpert, Inc.
> A Cisco Learning Partner - We Accept Learning Credits!
> Telephone: +1.810.326.1444
> Fax: +1.810.454.0130
>
> Mailto: mgreenlee@ipexpert.com
>
>
>
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>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Gary
> Duncanson
> Sent: Friday, November 16, 2007 4:59 PM
> To: ccielab@groupstudy.com
> Subject: Inverse Mask Odds and Evens Fun
>
> Folks,
>
> I have been looking at the ubiquitous odds and evens scenarios today using
> distribute lists. It's been sometime since I considered this one.
>
> I think I understand how an inverse mask of say 0.0.14.0 in the ACL that
> the
> distribute list calls can permit even routes in a contiguous range of
> networks..but can anyone clarify the process by which you correctly work
> out
> the inverse mask you need for any group of addresses you may have to work
> with?
>
> Hitting the archives again as you read this.
>
> Cheers
> Gary
>
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