Packet rules, which are the combination of IP filtering and network address translation (NAT), act like a firewall to protect your internal network from intruders.
IP filtering lets you control what IP traffic to allow into and out of your network. Basically, it protects your network by filtering packets according to rules that you define. NAT, on the other hand, allows you to hide your unregistered private IP addresses behind a set of registered IP addresses. This helps to protect your internal network from outside networks. NAT also helps to alleviate the IP address depletion problem, since many private addresses can be represented by a small set of registered addresses.