Network Security First-Step, Second Edition

Packet Filtering via ACLs

As you probably already know, all information that flows across the Internet uses TCP/IP and, in turn, this information is sent in small pieces known as packets. In the early days of the Internet, filtering based on packets was common and, in many cases, routers in many networks still use packet filtering. Packet filters are often used as a first defense in combination with other firewall technologies. Today, their most common implementation is seen in the ACLs of routers at the perimeters of networks.

Packet filtering is one of the oldest and most common types of packet inspection technologies available. It begins by inspecting a packet’s contents and applying rules to determine whether a packet should be dropped or ...