A firewall is a software or hardware system which enables a system to protect the data by selectively restricting the flow of incoming and outgoing traffic on a network. It allows the user to define security policies depending upon which the traffic is filtered. It isolates the private network from public network so that access to resources is controlled. A firewall works on different network layers. A firewall acting on one layer focuses on the policies assigned for it and is not concerned about its operation on other layers.