Firewalls can be categorized into three general classes: packet filters, stateful firewalls, and
application layer firewalls [5]. Each type provides a certain type of security and is best
described within the context of a network layer model—for example, the Open Systems
Interconnect (OSI) or TCP/IP model, as shown in Figure 8.2.
Recall that the TCP/IP model consists of four basic layers: data link, networking (IP),
transport (TCP and UDP), and application. Each layer is responsible for providing a certain
service to the layer above it. The first layer (data link) is responsible for transmitting
information across the local area network (LAN); examples include Ethernet and 802.11
networks. The network layer (routing, implemented IP) concerns routing information across