What is a firewall policy or ruleset?Ī firewall determines what packets should be accepted, denied, or dropped based on its policy. Unix and Linux users may be familiar with the iptables firewall. Unlike the real world, a dropped packet just ceases to exist.Ī firewall may be implemented as a hardware device (such a Linksys or Netgear firewall you can buy in an electronics store) or in software, such as the Windows Firewall or the MacOS Firewall. As in the real world, a denied packet is turned from the gate and sent back to where it came from.

A firewall functions analogously, looking at each packet of data to determine where it came from and where it is going, or both, and deciding if the packet should be accepted and allowed to continue on its way or if it should be denied or dropped. In the physical world, a guard might make decisions based on where a person is trying to go, where they came from, or both before admitting them. It protects the computer or network by limiting points of access and providing criteria that must be met before being allowed to enter. A firewall around a computer or network is like the wall around a castle or city.