Firewall is essential components of network security infrastructure designed to control and monitor incoming and outgoing network traffic based on predetermined security rules. There are several types of firewalls, each with its own method of operation and advantages. Here are some of the most common types of network firewalls:

1. Packet Filtering Firewall:
   • Packet filtering firewalls operate at the network layer (Layer 3) of the OSI model.
   • They examine packets as they pass through the firewall and determine whether to allow or block them based on predefined rules.
   • Rules are typically based on information such as source and destination IP addresses, port numbers, and protocol types.
   • Packet filtering firewalls are often implemented in routers or as software on dedicated firewall appliances.
   • Example: iptables in Linux is a packet filtering firewall that allows administrators to define rules for filtering network traffic based on various criteria like source IP, destination IP, port number, etc. Cisco’s Access Control Lists (ACLs) are commonly used packet filtering mechanisms in Cisco routers and switches.
2. Stateful Inspection Firewall:
   • Stateful inspection firewalls operate at both the network layer (Layer 3) and the transport layer (Layer 4).
   • In addition to filtering packets based on static criteria like packet headers, stateful inspection firewalls maintain a state table that tracks the state of active connections.
   • These firewalls allow incoming packets only if they match an existing connection in the state table.
   • Stateful inspection firewalls provide better security and performance compared to packet filtering firewalls.
   • Example: Check Point Firewall is a commercial firewall solution that employs stateful inspection to monitor and control network traffic based on connection states.
3. Proxy Firewall (Application-Level Gateway):
   • Proxy firewalls operate at the application layer (Layer 7) of the OSI model.
   • Instead of allowing direct connections between network hosts, proxy firewalls act as intermediaries, receiving and forwarding network traffic on behalf of the client.
   • Proxy firewalls inspect and filter both incoming and outgoing traffic at the application layer, providing granular control over protocols and applications.
   • They offer enhanced security by hiding internal network details and performing deep packet inspection.
   • Example: Squid Proxy Server is a widely used open-source proxy server that can be configured as a firewall to intercept and filter HTTP, HTTPS, FTP, and other application-layer protocols.
4. Next-Generation Firewall (NGFW):
   • Next-generation firewalls combine traditional firewall functionalities with advanced features such as intrusion prevention, application awareness, deep packet inspection (DPI), and integration with threat intelligence feeds.
   • NGFWs analyse network traffic at multiple layers of the OSI model, allowing for more sophisticated threat detection and prevention capabilities.
   • They provide enhanced visibility into application usage and user behaviour, enabling better control over network activities.
   • Example: Palo Alto Networks’ Palo Alto Networks’ NGFW series offers advanced threat prevention capabilities, application-based policies, and integration with threat intelligence services to protect networks from sophisticated cyber threats.

Each type of firewall has its strengths and weaknesses, and the choice of firewall depends on the specific security requirements and network architecture of an organization.