What is a firewall? How does it work?

What is a firewall? How does it work?

A firewall is a system or group of systems that enforces an access control policy between two networks. The actual means by which this is accomplished varies widely, but in principle, the firewall can be thought of as a pair of mechanisms: one, which exists to block traffic, and the other, which, exists to permit traffic. Some firewalls place a greater emphasis on blocking traffic, while others emphasize permitting traffic. Probably the most important thing to recognize about a firewall is that it implements an access control policy. 

Types of Firewalls
Following are the types of firewalls based on how they work:

  • Packet Filtering Firewalls - that block selected network packets.
  • Proxy Servers - that make network connections for you.
  • Unified threat management (UTM) firewall - A UTM device typically combines, in a loosely coupled way,
    the functions of a stateful inspection firewall with intrusion prevention and antivirus and simple malware
    detection. It may also include additional services for content filtering. 
  • Next-generation firewall (NGFW) - Firewalls have evolved beyond simple packet filtering and stateful

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