Palomuuri

Palomuuri, Finnish for firewall, is a device, software, or combination of both that monitors and controls the flow of network traffic between computer networks according to a defined security policy. Its primary purpose is to create a barrier that separates trusted internal networks from untrusted external networks, such as the internet, while allowing legitimate communication to pass.

Firewalls can operate at various layers of the network stack and enforce rules based on source and destination IP addresses, ports, and protocols, as well as more advanced attributes such as connection state or application type. Stateless (packet-filtering) firewalls examine each packet in isolation, while stateful firewalls track active connections and make decisions based on the state of those connections. Proxy-based firewalls relay traffic through an application-level proxy, applying content inspection and access controls before forwarding the request. Modern next-generation firewalls combine multiple techniques and often include integrated intrusion prevention, application awareness, and user identity features.

Common types include network firewalls deployed at network perimeters, host-based firewalls installed on individual devices, and cloud or virtual firewalls designed for cloud environments. Some firewalls also offer network address translation (NAT), virtual private network (VPN) support, and deep packet inspection.

Deployment considerations include placement within the network, the default-deny principle, rule-set management, performance, and logging. Firewalls require ongoing configuration maintenance to reflect changing security requirements and to avoid misconfigurations that could block legitimate traffic or create gaps.