Firewalls are network security devices that monitor and control incoming and outgoing network traffic based on a set of security rules. They act as a barrier between internal trusted networks and untrusted outside networks such as the public internet.

Firewalls inspect all traffic that attempts to flow in and out of a network using stateful packet inspection. They determine whether to allow or block specific traffic based on factors like source and destination IP address, port number, protocols and packet content. They provide an essential layer of protection against cyber threats by shielding internal systems and data from unauthorized access.

Firewalls are a critical component of network perimeter security. By selectively allowing traffic and blocking threats, they help enforce security policies, prevent intrusions, and protect internal users, data and resources.