A firewall is a network security device that grants or rejects network access to traffic flows between an untrusted zone (e.g., the Internet) and trusted zone (e.g., a private or corporate network). The firewall acts as the demarcation point or “Immigration officials” in the network, as all communication should flow through it and it is where traffic is granted or rejected access. Firewalls enforce access controls through a positive control model, which states that only traffic defined in the firewall policy is allowed onto the network; all other traffic is denied.