Applying Access Lists to Interfaces

When the access list is created after that it has to be applied to an interface to perform its function of filtering traffic. There are two types of access lists, out of which only one can be applied at a time. One is inbound and the other is outbound.

Inbound:

An inbound access list only controls that type of traffic which is allowed to enter the router through a particular interface.

Outbound:

As an access list is applied to a particular interface as an outbound, then it is checked when the data package leaves the boundary of the network
If our router has three interfaces (E0, S0, and S1) and each is connected to a different system.

•    When the host 192.168.15.30 recognizes that the server wants to correspond with the remote network, and then it will forward the packet to router line S1.
•    As the router is directed to do it will consult its routing table to determine that these packets should be forwarded or not to E0.
•    The interface E0 carries an outbound access list. This list will be matched for all the data packages that want to leave the router. If the package matches the list than it is free to go.

It is in front of you that one simple access list can have multiple consequences. The outbound access list applied to interface E0 blocked traffic coming from the network 192.168.15.0 to reach the network 192.168.35.0. It means that the whole traffic is blocked even when it is requested by the host.

The access list applied to an interface will not have any impact on the traffic that comes from the router where the access lists are defined. To understand this, here is an example, when we configure an outbound access list that also denies telnet traffic will not deny a telnet session start from the router to exit that crossing point.