Firewalls and Internet Security: Repelling the Wily Hacker, Second Edition

Chapter 9. Kinds of Firewalls

	fire wall noun: A fireproof wall used as a barrier to prevent the spread of a fire.
	--AMERICAN HERITAGE DICTIONARY

Some people define a firewall as a specific box designed to filter Internet traffic—something you buy or build. But you may already have a firewall. Most routers incorporate simple packet filters; depending on your security, such a filter may be all you need. If nothing else, a router can be part of a total firewall system—firewalls need not be one simple box.

We think a firewall is any device, software, or arrangement or equipment that limits network access. It can be a box that you buy or build, or a software layer in something else. Today, firewalls come “for free” inside many devices: routers, modems, ...