FIGURE 5 . 1 IP access list example with three LANs and a WAN connection
At this point, the access list is configured to deny source addresses from the Sales LAN access to the Finance LAN and allow everyone else. But remember, no action will be taken until the access list is applied on an interface in a specific direction. But where should this access list be placed? If you place it as an incoming access list on E0, you might as well shut down the Ethernet interface because all of the Sales LAN devices will be denied access to all networks attached to the router. The best place to apply this access list is on the E1 interface as an outbound list:
Lab_A(config)#int e1
Lab_A(config-if)#ip access-group 10 out
This completely stops traffic from 172.16.40.0 from getting out Ethernet 1. It has no effect on the hosts from the Sales LAN accessing the Marketing LAN and the Internet since traffic to those destinations doesn’t go through interface E1. Any packet trying to exit out E1 will have to go through the access list first. If there were an inbound list placed on E0, then any packet trying to enter interface E0 would have to go through the access list before being routed to an exit interface.
Let’s take a look at another example of a standard access list. Figure 7.2 shows an internetwork of two routers with three LANs and one serial WAN connection.
You want to stop the Accounting users from accessing the Human Resources server attached to the Lab_B router but allow all other users’ access to that LAN. What standard access list would you create and where would you place it?