Access Control List is an important function a router can perform. The router can act as a firewall. In doing so it must make decisions based on information provided. In the case of the Router it is an Access Control list.

The Access Control List uses a mask of 0, 255, or a specific identifier to determine if traffic is considered to be interesting. Interesting traffic is packets that will be sent to the proper destination. Not interesting traffic is simply discarded.

The Access Control List can make decisions base on an IP address, a range of IP addresses, a protocol, or even a port number.

Access lists can be standard or extended. The extended obviously being able to resolve more granularity than the standard list.

The access lists provide the mechanism where Instructors can have access to student computers while students computers are blocked from accessing the Instructor computers or any other nodes that may need security.

Access lists can also be utilized to provide for secure data transfer of critical data such as grades that malicious students would be eager to destroy or modify.