Security rules: ACLs, implicit deny, URL and content filtering, zones and screened subnets

Security rules decide what traffic is permitted between parts of a network. They appear as access control lists on routers and switches, as policies on firewalls and cloud security groups, and as filters on web proxies and secure web gateways. Writing them correctly, and understanding exactly how they are processed, is essential for both the exam and real life, because a single rule in the wrong place can either expose a server to the internet or cut off an entire office.