The firewall market is one of the largest and most mature security markets. Firewalls have undergone several stages of development, from early packet filtering and circuit relay firewalls to application layer (proxy-based) and dynamic packet filtering firewalls. Throughout their history, however, the goal has been to enforce an access control policy between two networks and thus should be viewed as an implementation of policy. A firewall is a mechanism used to protect a trusted network from an untrusted network while allowing authorized communications to pass from one side to the other.