The firewall is a process that looks for signs of an attack if the network is being breached. It monitors signals from various traffic sources and may be alerted to an attack.

Steven Bellovin invented the word "firewall" to describe the process of filtering traffic between secure networks and the internet. He used the word as a metaphor, suggesting firewalls are like partitions to prevent a fire from spreading throughout a building. A firewall does this by filtering incoming and outgoing traffic and only allowing certain things to pass.

Along the way, firewalls have been transformed into different types of firewalls in response to the evolution of digital threats.

There are five types of firewalls: Network, Host, Anti-malware, Application, and Other. However, it doesn't matter how many different types there are because they each do something different. In addition, the specific number isn't essential because enterprises need more than one type of firewall to protect their systems.

Five types of firewall include:
  • packet filtering firewall
  • circuit-level gateway
  • application-level gateway (aka proxy firewall)
  • stateful inspection firewall
  • next-generation firewall (NGFW)