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XOR for Computing ACL Masks
Hi Brian, I do not see any problem with that at all
Xor is only 1 (true) if 1 Xor 0; 0 Xor 1
http://mathworld.wolfram.com/XOR.html http://www.vipan.com/htdocs/bitwisehelp.html http://en.wikipedia.org/wiki/Xor_swap_algorithm http://en.wiktionary.org/wiki/XOR
>From the Internetwork Link:
10.0.0.0/16 10.4.0.0/16 10.32.0.0/16 10.36.0.0/16
128 64 32 16 8 4 2 1 0 0 0 0 0 0 0 0 0 4 0 0 0 0 0 1 0 0 32 0 0 1 0 0 0 0 0 36 0 0 1 0 0 1 0 0 ================================================= AND 0 0 0 0 0 0 0 0 == 0 XOR 0 0 1 0 0 1 0 0 == 36
So the access-list should be something like 10.0.0.0 0.36.0.0 You can test this also in the router using
R6#show ip route 10.0.0.0 255.219.0.0 long | b Gate Gateway of last resort is not set 10.0.0.0/16 is subnetted, 4 subnets C 10.0.0.0 is directly connected, Loopback101 C 10.4.0.0 is directly connected, Loopback101 C 10.32.0.0 is directly connected, Loopback101 C 10.36.0.0 is directly connected, Loopback101
HTH Victor.-
—–Mensaje original—– De: nobody@groupstudy.com [mailto:nobody@groupstudy.com] En nombre de Brian Enviado el: Viernes, 28 de Julio de 2006 11:32 a.m. Para: ccielab@groupstudy.com Asunto: XOR for Computing ACL Masks
I have been looking at the following example on how to calculate an appropriate mask. http://www.internetworkexpert.com/resources/01700370.htm. While I understand the process that was use (and it works well) I cannot figure out how the mask was calculated using XOR in example 2. If I understand XOR correctly anytime there are an even number of 1’s in a computation the result of XOR will be 0. When there are an odd number of 1’s the result will be 1. This does not seem to hold true in the example. Can someone please help me understand how XOR was applied to come up with the mask in example 2. Specifically the second octet is where the confusion comes in. The only conclusions I can draw are that this is not truly an XOR or I do not understand XOR. By the way I tried to XOR 0011 in the MS calculator and the result is 0. Thanks!
