What is firewall?

In Information technology industry all the networks in the worlds tends to connect each other through world wide web (internet), in today's scenarios it is mandatory for each network of every business establishment to pose its private network (Intranet LAN) to more risk prone internet to grow its business through online publishing and marketing. here is risk for organisation or any individual connected to internet and not bothering about what type of communication or data transfer happening internally. so, firewall is a mechanism which keeps tracks of any communication between its private network(LAN, Intranet etc)  and the public network (internet).

Firewall is computer hardware or software base network security system which controls incoming or outgoing network traffic by analyzing the data packets and determining whether they should be allowed through or not, based on a rule set defined.A firewall establishes a barrier between a trusted, secure internal network and another network (e.g. Internet) that is not assumed to be secure and trusted. 

Firewalls are termed as  network devices which enforce an organization's security policy, different organisation can implement network security policy in the firewall system with various methods, . These methods filter network tra ffic at one or more of the seven layers of the ISO network model, most commonly at the application, transport, and network, and data-link levels. In addition, researchers have developed some newer methods, such as protocol normalization and distributed firewalls though they are not popular yet.