Routers and stateless firewalls (or packet-filtering firewalls) perform basic filtering with an access control list (ACL). ACLs identify what traffic is allowed and what traffic is blocked. An ACL can control traffic based on networks, subnets, IP addresses, ports, and some protocols. Implicit deny blocks all access that has not been explicitly granted. Routers and firewalls use implicit deny as the last rule in the access control list. Antispoofing methods block traffic using ACL rules.
