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Firewalls protect a network against threats such as malware, ransomware, viruses, worms, spyware and phishing. They monitor network traffic and can prevent unauthorized data access from outside and from within the organization’s internal networks. Firewalls help you control how your computers use bandwidth, ensuring that critical applications are prioritized over non-essential ones.

Firewall technology has evolved over time to meet the changing needs of networks. While different types of firewalls incorporate varied methods of filtering, they all developed to surpass the capabilities of previous generations. They offer different levels of security and operate at a particular level of the standardized communications model, the Open Systems Interconnection (OSI).

Static packet-filtering firewalls, also known as layer 3 firewalls, check all individual data packets for their destination, source address and port numbers. They operate at a basic level to prevent two networks from directly connecting without permission.

Stateful inspection firewalls, or layer 4 firewalls, maintain a table that tracks existing connections. When a new data packet arrives, it compares the information in its header with the tables to determine if the packet is part of an established connection. If so, the firewall simply checks the existing records to see if it should be allowed through. If not, the packet is evaluated according to the rules for new connections.

Service access policies, a type of firewall design policy, dictate what types of traffic the firewall should permit and deny. The person who implements this design policy, sometimes called a firewall administrator, translates the policy into a series of technical statements that tell the firewall hardware or software what to do with incoming and outgoing traffic. The list of criteria the firewall uses is referred to as the firewall ruleset.