From: psimmons@xxxxxxxxxxxxxxxxxxx
Date: Fri Jan 19 2001 - 06:37:47 GMT-3
Chenriqu,
It's often easiest to write these things out in Binary, as it's
then simple to see the patterns forming. Look at what happens to
bit 24 in the following sequence.
00000000 00000000 11111110 11111111 = 0.0.254.255 - Mask
11100000 10101000 00000001 00000000 = 192.168.1.0 (Will Match)
11100000 10101000 00000010 00000000 = 192.168.2.0 (Won't Match)
11100000 10101000 00000011 00000000 = 192.168.3.0 (Will Match)
11100000 10101000 00000100 00000000 = 192.168.4.0 (Won't Match)
11100000 10101000 11111011 00000000 = 192.168.251.0 (Will Match)
11100000 10101000 11111100 00000000 = 192.168.252.0 (Wont Match)
11100000 10101000 11111101 00000000 = 192.168.253.0 (Will Match)
11100000 10101000 11111110 00000000 = 192.168.254.0 (Wont Match)
The key is the last bit of Octet 3, (Bit 24 of the address). If it
is set to 1, it will match the access-list (and therefore be
denied) if it is a 0, it won't match the access-list, and
(presumably there is a permit all) it will pass the access list.
In this example, the ZERO bits of the MASK are bits that must be
matched in the address being compared to the access-list statement, therefore a
s bit 24 of the address in the access list statement is set to one, all matchin
g addresses must have this bit set.
If the access list had read
access-list 10 deny 192.168.0.0 0.0.254.255
then all EVEN addresses would match the list, again because all even
addresses in this sequence have bit 24 of the address set to zero.
It doesn't matter in this case what any of the other bits in octet three
are set to, as they are in the "don't care" position of the mask.
Hope this helps!
Regards
Pete S.
<Brussels 21/22 June>
Original Message:
-----------------
From: chenriqu chenriqu@home.com
Date: Fri, 19 Jan 2001 02:52:40 -0500
Subject: BootCamp Lab3
Hello,
Can someone explain how Task Three question 4 works. I don't understand how
this access list allows only even networks (access-list 10 deny
192.168.1.0 0.0.254.255).
Any help would be greatly appreciated.
Thanks in advance
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