15.3. Firewall Technologies
There's 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 and DMZs and other firewall technologies to build a serious, formidable system fortress.
15.3.1. Access Control Lists (ACL)
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 ...