Firewalls

emphasis to the use of advanced authentication (see Chapter 3) and

the importance of policy (see Chapter 2 for determining how a firewall

will implement a protection scheme).

However, the burning question that needs to be answered before

we go any further is what really is a network firewall?

1.2 Firewall defined

A firewall is a system or group of systems that enforces an access

control policy between two networks, as shown in Figure 1.1. The

actual means by which this is accomplished varies widely, but in

principle, the firewall can be thought of as a pair of mechanisms: one

that exists to block traffic and the other to permit traffic. Some firewalls ...