What is a firewall and how does it protect networks?

A firewall is a network security device that monitors and controls incoming and outgoing network traffic based on predetermined security rules.\n\n**How firewalls work:**\n• **Traffic filtering** - Examines data packets and applies security rules\n• **Access control** - Allows or blocks traffic based on source, destination, port\n• **Network barrier** - Creates a protective boundary between trusted and untrusted networks\n• **Logging** - Records traffic patterns and security events for analysis\n\n**Types of firewalls:**\n\n**1. Packet filtering (Stateless):**\n• **Function** - Examines individual packets against static rules\n• **Criteria** - Source/destination IP, port numbers, protocol type\n• **Pros** - Fast processing, low resource usage\n• **Cons** - Cannot track connection state, limited security\n\n**2. Stateful inspection:**\n• **Function** - Tracks connection state and context\n• **Intelligence** - Remembers previous packets in the connection\n• **Security** - Better protection against sophisticated attacks\n• **Example** - Allows return traffic for established connections\n\n**3. Application layer (Proxy):**\n• **Function** - Inspects application-specific data and protocols\n• **Deep inspection** - Understands HTTP, FTP, SMTP content\n• **Security** - Highest level of protection and control\n• **Performance** - Slower due to detailed analysis\n\n**4. Next-generation firewalls (NGFW):**\n• **Features** - Combines traditional firewall with IPS, application awareness\n• **Intelligence** - User identity, application control, threat intelligence\n• **Integration** - Works with security information and event management (SIEM)\n\n**Firewall deployment:**\n• **Network perimeter** - Between internal network and internet\n• **Internal segmentation** - Between network zones (DMZ, servers, workstations)\n• **Host-based** - Software firewall on individual devices\n\n**Best practices:**\n• Default deny policy (block all, allow specific)\n• Regular rule review and cleanup\n• Monitor firewall logs for threats\n• Keep firmware updated\n• Test firewall rules and configurations