Dec
26

Access-list address and wildcard pair calculations are based
around the AND and XOR logic gates.

AND: The output is high only when both inputs A and B are high.

A AND B
______________
| A | B | out |
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
--------------

XOR: The output is high when either of inputs A or B is high, but not if
both A and B are high.

A XOR B
______________
| A | B | out |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
--------------

To find the most specific address and wildcard pair that will
match two addresses, A and B, we use the gates AND and XOR. The address
we will check in the access-list is A AND B. The wildcard used to check
in this list will be A XOR B.

access-list 1 permit [address_to_check] [wildcard_used_to_check]

Take the following example:

We have two IP addresses, 10.20.30.40, and 40.30.20.10. How do we
create an access-list that is the most specific match for these two
addresses? First, write both addresses out in binary:

10.20.30.40 = 00001010.00010100.00011110.00101000
40.30.20.10 = 00101000.00011110.00010100.00001010

To find the address_to_check, take the logical AND of these addresses.

   00001010.00010100.00011110.00101000
&& 00101000.00011110.00010100.00001010
--------------------------------------
00001000.00010100.00010100.00001000

This is our address_to_check: 8.20.20.8

To find the matching wildcard_used_to_check, we take the logical XOR of
these addresses.

    00001010.00010100.00011110.00101000
XOR 00101000.00011110.00010100.00001010
---------------------------------------
00100010.00001010.00001010.00100010

This is our wildcard_used_to_check: 34.10.10.34

Therefore, the most specific match for both 10.20.30.40 and 40.30.20.10
would be:

access-list 1 permit 8.20.20.8 34.10.10.34

Here's one more:

A = 1.2.3.4
B = 5.6.7.8

1.2.3.4 = 00000001.00000010.00000011.00000100
5.6.7.8 = 00000101.00000110.00000111.00001000

A && B = 00000001.00000010.00000011.00000000
A XOR B = 00000100.00000100.00000100.00001100

Therefore the access-list would read:

access-list 1 permit 1.2.3.0 4.4.4.12
Brian McGahan, CCIE #8593, CCDE #2013::13
About Brian McGahan, CCIE #8593, CCDE #2013::13

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