What Is a Firewall?
Firewall is a network security tool that monitors, filters, and controls incoming and outgoing data based on security policies. It creates a barrier between internal networks and untrusted external ones and protects against threats like malware, viruses, and spyware. Firewalls are deployed either in hardware as dedicated firewall appliances or in software form in firewall applications running on computers or servers.
A firewall checks outgoing data by scanning packets and comparing them against predetermined criteria. The rules determine whether to accept or deny a packet of data and are often based on the source, destination, and protocol type. This is called packet filtering. A more advanced approach, stateful inspection, goes further than packet filtering. It tracks the status of active connections and evaluates them in context to detect sophisticated threats. But stateful inspection takes a toll on performance.
Other firewall capabilities include device-based protection (often referred to as sandboxing) that runs important applications in isolated environments. This is designed to prevent backdoors that hackers might use to access the devices in your network, hijack them, and steal data or information. Firewalls also help to block unauthorized remote access from third parties to your network devices and systems by screening out users without proper authentication credentials.
Firewall selection depends on what you’re trying to protect—your whole network, specific devices, or individual apps. Choose between hardware-based, network-based, and host-based firewalls —and consider what filters you might need to support. A hardware-based firewall can be installed at the perimeter, or edge, of your network to protect you from hosts on the Internet; a network-based firewall is placed between internal network segments to separate corporate and residential systems, or research and marketing systems.