You might have locked the door to the computer center in which the servers are located, but the computers remain connected to the network. A network is another type of door, or rather a series of doors, that can allow data in or out. To provide services to your users, some of those doors must be open at least some of the time, but server administrators must make sure that only the right doors are left open.
A firewall is a software program that protects a computer or a network by allowing certain types of network traffic in and out of the system while blocking others. A firewall is essentially a series of filters that examine the contents of packets and the traffic patterns to and from the network to determine which packets they should allow to pass through.
The object of a firewall is to permit all of the traffic that legitimate users need to perform their assigned tasks yet block everything else. Note that when you are working with firewalls, you are not concerned with subjects like authentication and authorization. Those are mechanisms that control who is able to get through the server’s open doors. The firewall determines which doors are left open and which are shut tight.
Following are points which will cover in coming sections