Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 264 ICMP Access List Consider this scenario. You’ve been asked to block anyone from the 172.18.x.x network from “pinging” anyone on the 172.16.x.x network. You want to allow everything else, including all other ICMP packets. The specific ICMP port that a “ping” uses is echo. To block specific ICMP parameters, use an extended IP access list. On Router B, we would configure: Router(config)# access-list 102 deny icmp 172.18.0.0 0.0.255.255 172.16.0.0 0.0.255.255 echo Router(config)# access-list 102 permit icmp 172.18.0.0 0.0.255.255 172.16.0.0 0.0.255.255 Router(config)# access-list 102 permit ip any any The first line blocks only ICMP echo requests (pings). The second line allows all other ICMP traffic. The third line allows all other IP traffic. Don’t forget to apply it to an interface on Router B: Router(config)# int e0 Router(config-if)# ip access-group 102 in Untrusted networks (such as the Internet) should usually be blocked from pinging an outside router or any internal hosts: Router(config)# access-list 102 deny icmp any any Router(config)# access-list 102 permit ip any any Router(config)# interface s0 Router(config-if)# ip access-group 102 in The above access-list completed disables ICMP on the serial interface. However, this would effectively disable ICMP traffic in both directions on the router. Any replies to pings initiated by the Internal LAN would be blocked on the way back in. * * * 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 265 Telnet Access List We can create access lists to restrict telnet access to our router. For this example, we’ll create an access list that prevents anyone from the evil 172.18.x.x network from telneting into Router A, but allow all other networks telnet access. First, we create the access-list on Router A: Router(config)# access-list 50 deny 172.18.0.0 0.0.255.255 Router(config)# access-list 50 permit any The first line blocks the 172.18.x.x network. The second line allows all other networks. To apply it to Router A’s telnet ports: Router(config)# line vty 0 4 Router(config-line)# access-class 50 in * * * 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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