Firewalls

In computer terms, a firewall is a boundary system that sits between two networks and enforces a security policy that determines what information is allowed to pass between them. The networks in question are typically a corporate, or private, local area network (LAN) and the public Internet. The security policy can be very simple, allowing most communication to pass through, or can be very complex, allowing only specifically designated traffic from specifically designated hosts to cross the boundary.

A firewall acts like a security guard that monitors all incoming and outgoing traffic and makes decisions about whether or not certain traffic is allowed. These decisions are based on the security policy. Under the simplest, least restrictive security policy, everything is allowed except that which is explicitly denied. Under the most complex, most restrictive policy, everything is denied except that which is explicitly allowed...