Suggest an editImprove this articleRefine the answer for “What is a firewall?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)**A firewall** is a network security tool that controls and filters network traffic, deciding what can pass through and what must be blocked. **Key point:** a firewall is a basic building block of defense, not the only security measure.Shown above the full answer for quick recall.Answer (EN)Image**A firewall** is a network security tool that **controls and filters network traffic**, deciding **what can pass through and what must be blocked**. In simple terms: *a firewall is a "guard" between networks (for example, between the internet and the internal network).* --- ## Why a firewall is needed A firewall is needed to: - protect the network from unauthorized access; - block attacks and malicious traffic; - restrict access to services and ports; - separate networks (internal, external, DMZ). Without a firewall: - anyone can try to connect to the services; - the risk of breaches and attacks increases. --- ## How a firewall works A firewall analyzes every network packet and compares it to a set of rules: - **allow**, and the packet passes through; - **deny**, and the packet is blocked. What can be checked: - the source and destination IP address; - the protocol (TCP, UDP, ICMP); - the port number; - the connection state. The rules are applied automatically and continuously. --- ## Main types of firewall ### 1. Packet filtering firewall - analyzes individual packets; - works fast; - does not track connection state. Simple, but limited. --- ### 2. Stateful firewall - tracks connection state; - understands which traffic is a reply; - safer and smarter. **The most common type**. --- ### 3. Application firewall - analyzes traffic at the application level; - understands HTTP, FTP, SMTP and others; - protects against attacks on applications. Often used for web services. --- ## Where a firewall can be located - on a router; - on a dedicated server; - built into the operating system; - in the cloud infrastructure. A firewall can be **network-based** or **host-based**. --- ## What a firewall does NOT do It is important to understand its limitations: - it does not encrypt data; - it does not protect against all types of attacks; - it does not replace antivirus software and IDS/IPS. A firewall is a **basic building block**, not the only defense.For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.