Block illegal addresses at the outgoing interfaces. Besides preventing an attacker from using the router to attack other sites, it helps identify poorly configured internal hosts or networks. This approach may not be feasible for complicated networks. [RFC 2827]
fumtek(config)# no access-list 101
fumtek(config)# access-list 101 permit ip 14.2.6.0 0.0.0.255 any
fumtek(config)# access-list 101 deny ip any any log
fumtek(config)# no access-list 102
fumtek(config)# access-list 102 permit ip any 14.2.6.0 0.0.0.255
fumtek(config)# access-list 102 deny ip any any log
fumtek(config)# interface eth 1
fumtek(config-if)# ip access-group 101 in
fumtek(config-if)# exit
fumtek(config)# interface eth 0
fumtek(config-if)# ip access-group 101 out
fumtek(config-if)# ip access-group 102 in
4. Block packets coming from the outside (untrusted network) that are obviously fake or have source or destination addresses that are reserved, for example networks 0.0.0.0/8, 10.0.0.0/8, 169.254.0.0/16, 172.16.0.0/20, 192.168.0.0/16. This protection should be part of the overall traffic filtering at the interface attached to the external,