Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 268 Advanced Wildcard Masks Earlier in this section, we discussed the basics of wildcard masks. The examples given previously matched one of three things: • A specific host • A specific octet(s) • All possible hosts It is also possible to match groups or ranges of hosts with wildcard masks. For example, assume we wanted a standard access-list that denied the following hosts: 172.16.1.4 172.16.1.5 172.16.1.6 172.16.1.7 We could create an access-list with four separate lines: Router(config)# access-list 10 deny 172.16.1.4 0.0.0.0 Router(config)# access-list 10 deny 172.16.1.5 0.0.0.0 Router(config)# access-list 10 deny 172.16.1.6 0.0.0.0 Router(config)# access-list 10 deny 172.16.1.7 0.0.0.0 However, it is also possible to match all four addresses in one line: Router(config)# access-list 10 deny 172.16.1.4 0.0.0.3 How do I know this is correct? Let’s write out the above four addresses, and my wildcard mask in binary: 172.16.1.4: 10101100.00010000.00000001.00000100 172.16.1.5: 10101100.00010000.00000001.00000101 172.16.1.6: 10101100.00010000.00000001.00000110 172.16.1.7: 10101100.00010000.00000001.00000111 Wild Card Mask: 00000000.00000000.00000000.00000011 Notice that the first 30 bits of each of the four addresses are identical. Each begin “10101100.00010000.00000001.000001”. Since those bits must match exactly, the first 30 bits of our wildcard mask are set to 0. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 269 Advanced Wildcard Masks (continued) Notice now that the only bits that are different between the four addresses are the last two bits. Not only that, but we use every computation of those last two bits: 00, 01, 10, 11. Thus, since those last two bits can be anything, the last two bits of our wildcard mask are set to 1. The resulting access-list line: Router(config)# access-list 10 deny 172.16.1.4 0.0.0.3 We also could have determined the appropriate address and wildcard mask by using AND/XOR logic. To determine the address, we perform a logical AND operation: 1. If all bits in a column are set to 0, the corresponding address bit is 0 2. If all bits in a column are set to 1, the corresponding address bit is 1 3. If the bits in a column are a mix of 0’s and 1’s, the corresponding address bit is a 0. Observe: 172.16.1.4: 10101100.00010000.00000001.00000100 172.16.1.5: 10101100.00010000.00000001.00000101 172.16.1.6: 10101100.00010000.00000001.00000110 172.16.1.7: 10101100.00010000.00000001.00000111 Result: 10101100.00010000.00000001.00000100 Our resulting address is 172.16.1.4. This gets us half of what we need. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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Cisco CCNA Study Guide - Router Alley

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