To add to this, and looking at it from a few steps back, one can summarize the base functionality of a firewall as something which "sits in between various areas of a network (or networks) with differing levels of trust and enforces the semantics of these levels".
//umar. > A firewall is not so much a product or a feature as an architecture. You can > build a firewall on one system, or you can build it out of a number of > systems. -- SLUG - Sydney Linux User's Group - http://slug.org.au/ More Info: http://lists.slug.org.au/listinfo/slug
