What Is a Firewall?
A firewall is a hardware or software system that creates a protective barrier between a trusted internal network and untrusted external networks, such as the Internet. It monitors incoming and outgoing data packets and blocks or inspects them for dangerous activity. Firewalls work according to a list of predefined rules that determine whether a packet can enter or exit a network. They are like security guards that know all that goes on inside the building (network) they’re protecting and verify the safety of everyone who attempts to come in or out.
Firewalls can be categorized by what they protect and how they protect it. Network-based firewalls guard entire networks and are often hardware; host-based firewalls guard individual devices — known as hosts — and are generally software. Packet filtering firewalls examine data packets in isolation without knowledge of their context; stateful inspection firewalls compare individual packets to previously accepted connections; and proxy firewalls, also called application-level gateways, examine the payload or content of individual packets, which is more difficult for attackers to conceal.
Many firewalls offer flexible configuration options that allow you to set their behavior based on your security needs. For example, some configure the firewall to be permissive by default and block only specific traffic types; others use a restrictive configuration that automatically drops all traffic unless explicitly allowed. Keeping your firewall configured appropriately helps ensure that it can detect current malicious threats and respond to them quickly. You should also keep your firewall rules optimized to minimize the number of them, which can slow down processing and reduce performance.