Firewall Technologies

There are a whole bunch of firewall technologies, and they all differ in the way that they restrict information flow. Things like access control lists and dynamic packet filtering are often used as firewalls in their own right, or they can be implemented along with proxies, DMZs, and other firewall technologies to build a serious, formidable system fortress.

Access Control Lists

The first line of defense for any network that’s connected to the Internet is what we call access control lists (ACLs). These reside on your routers and determine by IP addresses which machines are allowed to use those routers and in what direction. ACLs have been around for decades and have some other really cool uses apart from being used as firewalls. Figure 15.2 gives you a great demonstration of how ACLs can work to prevent users on Network B from accessing Network A. However, hosts from Network B and can access Network A if a secure authenticated connection is used.

Figure...