What is a firewall?

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.