From: mgreenlee@ipexpert.com
Date: Sat Nov 17 2007 - 01:05:47 ART
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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