A router is used to connect networks and deliver packets based on the layer 3 IP addresses (also called the network layer).
ACL is a security feature of a router. ACL is set up to control traffic both incoming and outgoing. ACL tells the router to permit or allow traffic based on the following parameters:
- Source IP address
- Source subnet mask
- Destination IP address
- Destination subnet mask
Subnet masks are used to define different segments under a network. ACL can also control the traffic based on the protocol (TCP/UDP), port number, etc.
The primary reason to use an ACL is to provide a basic level of security for the network.