In the world of expensive commercial firewalls (the world in which I earn my living), the term "firewall” nearly always denotes a single computer or dedicated hardware device with multiple network interfaces. This definition can apply not only to expensive rack-mounted behemoths, but also to much lower-end solutions: network interface cards are cheap, as are PCs in general.
This is different from the old days, when a single computer typically couldn’t keep up with the processor overhead required to inspect all ingoing and outgoing packets for a large network. In other words, routers, not computers, used to be one’s first line of defense against network attacks.
Such is no longer the case. Even organizations with high capacity Internet connections typically use a multihomed firewall (whether commercial or open source-based) as the primary tool for securing their networks. This is possible, thanks to Moore’s law, which has provided us with inexpensive CPU power at a faster pace than the market has provided us with inexpensive Internet bandwidth. It’s now feasible for even a relatively slow PC to perform sophisticated checks on a full T1’s-worth (1.544 Mbps) of network traffic.
The most common firewall architecture one tends to see nowadays is the one illustrated in Figure 2-1. In this diagram, we have a packet-filtering router that acts as the initial, but not sole, line of defense. Directly behind this ...