Understanding the network.pdf - Back to Home

NOTE
In situations where you are filtering "internal" host access as an alternative to using
an ACL, it is possible to route undesirable packets to the null 0 interface. The null 0
interface is a pseudo-interface available on all Cisco routers. To route traffic to null,
use the command and replace the gateway address with null 0. Here
is a example that routes to null 0 any packet with a destination address using any of
the unregistered IP address spaces:
hobo(config)#ip route 10.0.0.0 255.0.0.0 null0
hobo(config)#ip route 172.16.0.0 255.266.0.0 null0
hobo(config)#ip route 192.168.0.0 255.255.0.0 null0
The in directional statement is not necessary, because all access control lists are
implicitly applied as inbound filters. Adding the in statement does not hurt, however,
and explicitly applying the filter direction is a good habit to get into when installing
access-groups on interfaces.
The activation of a dial-on-demand routing (DDR) interface is determined by certain
kinds of traffic the router sees as interesting. Different types of interesting traffic
(for each protocol, if needed) are defined with ACLs and applied to the DDR interface
with the interface configuration subcommand .
The dialer group number corresponds to the dialer-list. The dialer-list is created in
global configuration EXEC mode using the command.
IP access-lists are also used for traffic redirection through policy routing, and for
queuing lists through the route-map configuration subcommand .
Creating Access-Lists
Now let's get our hands dirty and create some standard access-lists. We will start
creating a standard IP access-list. Standard ACLs are built line by line, using the
global configuration command :