A firewall is a system that enforces an access control policey between two networks-such as your private LAN & the unsafe,public internet.The firewall determines which inside service can be accessed from the outside & vice-versa.The actual means by which this is accomplished varies widely,but in principal,the firewall can be thought of as a pair of mechanism:one to block traffic & one to permit traffic.A firewall is more than the locked front door to your network it’s your security guard as well.

Basic Purpose of a Firewall

Basically,a firewall does three things to protect the network:

• It blocks incoming data that might contain a hacker attack.
• It hides information about the network by making it seem that all outgoing traffic originates from the firewall rather than the network,this is called network address translation.
• It screens outgoing traffic to limit internet use or access to remote sites.