ABSTRACT
As shown in Figure 18.1, firewalls are used to control access to an internal network from anything that emanates from the public Internet. A firewall builds a blockade between an internal network that is assumed to be secure and trusted, and the Internet, that is not assumed to be secure and trusted [1]. A firewall is used to prevent risks such as (1) internal host systems exposure to inherently insecure Internet protocols and corresponding services, and (2) probes and attacks launched from hosts on the Internet. Without a firewall, network security becomes the sole responsibility of each host on the internal network as well as with large networks, and this scenario is simply not manageable. A firewall as shown in Figure 18.1 prevents unauthorized communication into or out of the network, and allows an organization to enforce a network security policy on traffic flowing between its network and the Internet. The most widely deployed firewall configurations are the three-legged firewall with the demilitarized zone (DMZ) outlined in Figure 18.1, the dual-homed firewall shown in Figure 18.2, host firewalls, and subnet firewalls. Within these structures, the firewall allows or blocks traffic based upon the IP address and port number.
