What is Firewall and How It Works
Apps & Software

What is Firewall and How It Works


If you use the Internet on your computer regularly, you must have come across a term called “Firewall.” We often see when our windows suggest turning on Firewall or antivirus firewall warning. Sometimes the software says that Firewall is blocking the connection. These are a few examples when seeing the word “Firewall” that is related to computers and the Internet. In this detailed post, I will try to explain what Firewall is and how a firewall works to protect our computers.

What is Firewall?

Firewalls are a barrier between your computer and the Internet that filters suspicious requests and intruders. A firewall can either a software program or a hardware that tries to keep your computer secure from intruders and suspicious programs trying to access your computer from the network. Many operating systems have a built-in software firewall to protect the computer from threats.

The firewall also aims to give control to the system user or network admin over incoming and outgoing network traffic by analyzing packets. If a network admin wants to block some specific kind of packets, he can define it in the firewall.

For example, A company has an internal network. The administrator of the company does not want their employees to connect to some specific websites and restrict file transfer from the Internet. They can use a firewall to deny these things easily.

How firewall works

Firewalls work by monitoring and controlling network traffic based on a set of predefined security rules. Their primary objective is to enforce security policies and protect a network from unauthorized access, malicious activities, and potential threats. Here’s a general overview of how firewalls work:

Traffic Analysis: Firewalls examine network traffic, analyzing packets of data as they pass through the network. They inspect the header and payload of each packet to gather information about the source and destination IP addresses, port numbers, protocols, and other relevant data.

Rule-Based Filtering: Firewalls use a set of predefined rules to determine whether to allow or block network traffic. These rules can be based on various criteria, such as IP addresses, port numbers, protocols, and specific keywords or patterns in the packet content. The firewall compares the characteristics of each packet against its rule set to decide whether to permit or deny the traffic.

Packet Filtering: In the case of packet-filtering firewalls, each packet is evaluated individually. The firewall checks whether the packet meets the specified criteria defined in the rules. If a packet matches an allowed rule, it is allowed to pass through the firewall. If a packet violates any rule, it is either blocked or dropped, depending on the firewall’s configuration.

Stateful Inspection: Stateful inspection firewalls maintain a state table that keeps track of ongoing network connections. They examine the context and state of the traffic, such as the connection establishment, the sequence of packets, and the acknowledgment of data. By comparing incoming packets to the state table, the firewall can make more intelligent decisions, allowing only legitimate traffic that belongs to established and authorized connections.

Application Layer Analysis: Firewalls can perform deep packet inspection at the application layer to analyze the content and behavior of network traffic. This allows them to identify specific applications or protocols, detect anomalies, and enforce stricter security policies. Application-level gateways or proxy firewalls often employ this technique to provide advanced security features, such as content filtering and application-aware filtering.

Logging and Reporting: Firewalls often maintain logs of the network traffic they process. These logs record details about allowed and denied connections, intrusion attempts, and other security-related events. Administrators can review these logs to investigate security incidents, analyze network behavior, and generate reports for compliance and auditing purposes.

Why Do We Need Firewalls?

Firewalls play a crucial role in network security and are essential for several reasons. Firewalls act as a barrier between internal networks and the external network, typically the Internet. They prevent unauthorized access and protect the internal network infrastructure, including servers, workstations, and other devices, from malicious actors and potential threats. Firewalls enforce access control policies, allowing organizations to define who can access their network and what services are permitted. By setting up rules and filters, firewalls can block unauthorized or suspicious incoming and outgoing traffic, reducing the attack surface and minimizing the risk of intrusions. It also helps mitigate various network threats, including unauthorized access attempts, malware infections, distributed denial-of-service (DDoS) attacks, and intrusion attempts. Firewalls provide visibility into network traffic by generating logs and reports.

Types of Firewall

Firewalls are network security devices that monitor and control incoming and outgoing network traffic based on predetermined security rules. They act as a barrier between an internal network and the external network, typically the Internet, to protect against unauthorized access and potential threats. There are several types of firewalls, including:

  1. Packet-filtering Firewalls: These are the most basic type of firewalls and operate at the network layer of the OSI model. They examine individual packets of data and filter them based on specified criteria, such as source and destination IP addresses, port numbers, and protocols. Packet-filtering firewalls can be either stateless or stateful.
  2. Stateful Inspection Firewalls: Also known as dynamic packet-filtering firewalls, these firewalls operate at the network layer and maintain a state table that keeps track of the ongoing connections. They inspect the characteristics and context of the network traffic, such as the state of the connection and the sequence of packets, to make more intelligent filtering decisions.
  3. Application-level Gateways (Proxy Firewalls): These firewalls operate at the application layer of the OSI model and act as intermediaries between clients and servers. They establish separate connections with both the client and server and inspect the application-layer traffic passing through them. Proxy firewalls offer enhanced security by analyzing the content of the packets and can provide additional services like caching and content filtering.
  4. Circuit-level Gateways: Also known as circuit-level proxies, these firewalls work at the session layer of the OSI model. They validate TCP handshakes between connections and create a virtual circuit for the transmission of data. Circuit-level gateways do not inspect the actual content of the packets but focus on ensuring the legitimacy of the connections.
  5. Next-Generation Firewalls (NGFWs): NGFWs combine traditional firewall functionality with additional advanced features such as deep packet inspection, intrusion prevention systems (IPS), application awareness, and user identity tracking. They provide a higher level of security and granular control by understanding the context and content of network traffic.
  6. Unified Threat Management (UTM) Firewalls: UTM firewalls integrate multiple security features into a single device. They typically combine firewall capabilities with antivirus, intrusion detection and prevention, content filtering, virtual private network (VPN) support, and other security features. UTM firewalls offer a comprehensive security solution for small to medium-sized enterprises.
  7. Virtual Firewalls: Virtual firewalls, also known as cloud firewalls, are designed specifically for virtualized environments or cloud computing. They provide network security and segmentation within virtual machines or cloud instances.

Deepanker Verma is the founder of Nerdy Guides. He is a tech blogger, software developer, and gadget freak. He also runs TheWPGuides and Techlomedia.

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